What Is the Stock Market — and How Does It Actually Work?

On 19 October 1987 — "Black Monday" — the Dow Jones Industrial Average fell 508 points, or 22.6%, in a single session, still the largest one-day percentage fall in the index's history. On 16 March 2020, as COVID-19 lockdowns spread, the Dow fell about 12.9%. Yet within two years of Black Monday US stocks were above their pre-crash highs, and the S&P 500 erased its entire pandemic loss by August 2020.

How can a system lose a fifth of its value in a day and then recover? Who actually sets the prices? What does it mean when "the market" goes up? This article explains the machinery: what a share is, how exchanges and order books work, what moves prices, what indices measure, why crashes happen, and what regulators do. It is an explanation of mechanics, not investment advice.

This is educational, not financial advice. Nothing here is a recommendation to buy, sell or hold any security. Past market behaviour does not predict future results. For decisions about your own money, consult a licensed, independent financial professional.

Key takeaways

• A share is a fractional ownership claim on a company: a residual claim on its profits and assets after creditors are paid.
• Exchanges match buy and sell orders; the price at any moment is simply where the most recent buyer and seller agreed.
• Prices move when expectations change — about future profits, risk and interest rates.
• Indices such as the S&P 500 summarise many stocks at once; "the market rose 1%" usually means an index rose 1%.
• Crashes typically combine stretched valuations, borrowed money (leverage) and panic selling. Safeguards such as circuit breakers were added after 1987.
• The US Securities and Exchange Commission (SEC), created in 1934, exists to protect investors, keep markets fair, orderly and efficient, and facilitate capital formation.

What a stock actually is

A company that needs money to grow has two broad options. It can borrow — take a bank loan or issue bonds — and promise to repay with interest. Or it can sell part of itself: equity. Lenders get a fixed claim; owners get whatever is left over after everyone else is paid.

A share of stock is one slice of that ownership. Suppose a company has 1 billion shares outstanding and you own 1 million of them. You own 0.1% of the company: in principle you are entitled to 0.1% of any dividends it pays, a proportional vote on matters put to shareholders, and 0.1% of whatever is left if the company is wound up after its debts are settled.

Shareholders can earn returns in two ways:

1. Dividends — cash the company chooses to distribute from profits.
2. Capital gains — the market price of the shares rising above what was paid.

Because shareholders are paid last, they bear more risk than lenders. In a bankruptcy they can lose everything. In a successful company their upside is uncapped. That asymmetry — limited downside to the amount invested, unlimited upside — is the core economic character of equity, and the reason it is contrasted with bonds in our explainer on stocks, bonds and investing basics.

Common and preferred shares

Most traded shares are common stock, which usually carries voting rights. Preferred stock typically has no or limited voting rights but gets priority over common shareholders for dividends and in liquidation. Some companies also issue multiple share classes with different voting power, which lets founders keep control while selling economic ownership.

A very short history

Shares are older than stock exchanges as we know them. The Dutch East India Company (VOC) sold shares to the public in 1602 and allowed them to be transferred, so investors could sell their stake to someone else rather than wait for the company's voyages to end. Trading in Amsterdam began in the open air and moved into a purpose-built exchange building in 1611. Historians note that tradable government debt existed earlier in Italian city-states, so "first stock market" is a popular label rather than an undisputed one.

In New York, 24 brokers signed the Buttonwood Agreement on 17 May 1792, agreeing to trade with one another and to set minimum commissions; the New York Stock Exchange counts that date as its founding. The body reorganised in 1817 and took the name New York Stock Exchange in 1863.

The modern US regulatory framework came out of the 1929 crash and the Great Depression. The Securities Act of 1933 required companies selling securities to the public to disclose information, and the Securities Exchange Act of 1934 created the SEC to oversee exchanges, brokers and ongoing disclosure.

The primary and secondary markets

There are really two markets inside "the stock market".

The primary market is where companies sell new shares and actually receive money. The best-known example is an initial public offering (IPO), when a private company first lists its shares. Investment banks usually help set the offer price and distribute the shares. The World Federation of Exchanges (WFE) counted 1,471 IPOs worldwide in 2025.

The secondary market is where investors trade existing shares with each other. When you buy a share on an exchange, your money goes to the seller, not to the company. Almost all of the daily trading reported in the news is secondary-market activity. The secondary market still matters to companies: a liquid market makes their shares more attractive, which lowers the cost of raising new equity later, and the share price affects everything from executive pay to the cost of acquisitions.

How a trade actually happens

The order book

Modern exchanges are electronic matching engines. Each listed stock has an order book — a list of standing buy orders (bids) and sell orders (asks or offers) at different prices.

• The highest bid is the most anyone is currently willing to pay.
• The lowest ask is the least anyone is currently willing to accept.
• The gap between them is the bid-ask spread.

A trade happens when a new order crosses that gap. A market order says "buy (or sell) now at the best available price"; it executes immediately against the best opposing order. A limit order says "buy only at this price or lower" (or "sell only at this price or higher"); if it cannot execute immediately, it rests in the order book until it can, or until it is cancelled.

Market makers and liquidity

Market makers are firms that continuously post both bids and asks, standing ready to buy or sell. They earn the spread in return for supplying liquidity — the ability for others to trade quickly without moving the price much. Heavily traded large-company stocks typically have very narrow spreads; thinly traded small companies can have wide ones.

A worked example

Imagine a stock's order book looks like this:

Bids (buyers)	Price	Asks (sellers)
	$50.03	400 shares
	$50.02	200 shares
300 shares	$50.00
500 shares	$49.99

The best bid is $50.00 and the best ask is $50.02, so the spread is 2 cents. If someone sends a market order to buy 300 shares, 200 fill at $50.02 and the remaining 100 at $50.03. The "last price" quoted on screens becomes $50.03. Nothing about the company changed — the price moved because a buyer was willing to pay up for immediate execution. Scale that across millions of orders and you have the minute-by-minute movement of a stock.

Clearing and settlement

Agreeing a trade is not the end of the process. Clearing confirms the details and nets obligations; settlement is the actual exchange of cash for shares. In the US, the SEC shortened the standard settlement cycle for most securities from two business days (T+2) to one (T+1), effective 28 May 2024. Shorter settlement reduces the time during which either side is exposed to the other failing to deliver.

What makes prices go up or down

At the deepest level, a share is worth the present value of the cash it is expected to generate for its owners in the future. Every price is therefore a forecast, and prices move whenever forecasts change. The main inputs are:

• Expected profits. When a company reports earnings above expectations, or signals stronger future growth, buyers tend to bid the price up. The reverse also holds. Note the word expectations: a company can report record profits and see its shares fall if investors had expected even more, a pattern we explore in why stock prices move after earnings.
• Interest rates. Future cash is discounted back to today using an interest rate. When rates rise, a dollar of profit in ten years is worth less today, and safer alternatives such as government bonds become more attractive, so stock valuations tend to fall. That is why central bank decisions move markets; see why interest rates affect everything.
• Risk. Investors demand compensation for uncertainty. When the outlook becomes more uncertain — a recession, a war, a financial crisis — the extra return investors require rises and prices fall even if profit forecasts have not yet changed.
• Inflation. Rising prices can lift companies' nominal revenues but also push up costs and interest rates. The net effect depends on whether companies can pass costs on. Our explainer on how inflation works covers the macroeconomics.
• Flows, sentiment and momentum. In the short run, prices are also pushed around by who needs to buy or sell, by fear and enthusiasm, and by trend-following. These forces can carry prices well away from what fundamentals alone would suggest.

Valuation in one number: the P/E ratio

A common shorthand is the price-to-earnings (P/E) ratio: share price divided by earnings per share. A company with a $100 share price and $5 of annual earnings per share has a P/E of 20 — investors are paying $20 for each $1 of current annual profit. A high P/E usually means investors expect strong future growth, low risk, or both, or that the shares are expensive. A low P/E can mean the opposite, or a bargain. On its own, the ratio does not say which.

Who is in the market

Participant	What they do	Typical horizon
Individual (retail) investors	Buy and sell through brokers, often via retirement or savings accounts	Days to decades
Mutual funds and ETFs	Pool money from many investors; index funds track a benchmark, active funds try to beat one	Months to years
Pension funds and insurers	Invest to meet long-term obligations to retirees and policyholders	Decades
Hedge funds	Use a wide range of strategies, often including leverage and short selling	Days to years
Market makers and high-frequency traders	Post bids and asks, profit from spreads and short-lived price differences	Seconds or less
Companies themselves	Issue new shares, buy back existing ones	Varies

Short sellers deserve a note. Short selling means borrowing shares, selling them, and hoping to buy them back later at a lower price to return to the lender. It lets investors express a negative view, which can help prices reflect bad news, but losses are theoretically unlimited if the price rises, and regulators sometimes restrict it during crises.

How big are stock markets?

According to the World Federation of Exchanges, global equity market capitalisation — the total market value of listed companies — was about US$151.9 trillion at the end of 2025, up 18.5% from the end of 2024. Trading value rose 36.8% over the same year. Market capitalisation is simply share price multiplied by shares outstanding, added up across companies; it rises and falls with prices, so it is a snapshot, not a stock of cash anyone could withdraw. For a comparison against the size of national economies, see what GDP is and why it matters.

What market indices measure

When a news presenter says "the market was up 1% today," they almost always mean an index — a calculated average of a defined group of stocks.

Index	What it covers	How it is weighted
S&P 500	About 500 large US companies chosen by committee	By market capitalisation (bigger companies count more)
Dow Jones Industrial Average	30 large US companies	By share price (higher-priced shares count more)
Nasdaq Composite	All common stocks listed on the Nasdaq exchange; tech-heavy	By market capitalisation
FTSE 100 / DAX / Nikkei 225	Leading companies in the UK, Germany and Japan	Varies by index

Weighting matters. In a cap-weighted index, a handful of very large firms can dominate the day's move. In the price-weighted Dow, a company with a $500 share price moves the index more than a much larger company whose shares happen to trade at $50. That is one reason professionals usually prefer the S&P 500 as a gauge of the US market.

Indices also suffer from survivorship: companies that shrink or fail are dropped and replaced by successful ones, so an index's long-run history describes the winners. That is legitimate for an index fund that rebalances the same way, but it overstates what someone picking stocks at random would have earned. Our piece on survivorship bias explains the general trap.

The efficient market hypothesis

The efficient market hypothesis (EMH) is associated above all with Eugene Fama of the University of Chicago. Its central idea is that prices reflect available information quickly, so it is hard to earn consistently above-average returns by trading on information everyone else already has. Fama shared the 2013 Nobel Memorial Prize in Economic Sciences with Lars Peter Hansen and Robert Shiller "for their empirical analysis of asset prices".

The pairing was telling. The prize committee's summary of their work was that asset prices are largely unpredictable over days and weeks, yet broad movements over longer periods of three to five years show some predictability. Shiller, a founder of behavioural finance, argued that prices can drift far from fundamental values because investors are not always rational. Both views have strong evidence behind them, and markets appear to be efficient enough to be very hard to beat, yet not so efficient that bubbles never form.

The EMH is usually described in three forms:

• Weak form: past prices cannot be used to predict future prices.
• Semi-strong form: all publicly available information is already reflected in prices.
• Strong form: even private information is reflected — a version contradicted by the profits from illegal insider trading.

The practical evidence most often cited is the long-running SPIVA scorecard published by S&P Dow Jones Indices, which compares actively managed funds with their benchmarks. Its year-end 2024 US report found that, over the 15 years to December 2024, there was no fund category in which a majority of active managers beat their benchmark. That finding underlies the growth of low-cost index funds, which simply hold every stock in an index in proportion to its weight.

Why crashes happen

Crashes are rarely caused by one thing. The recurring ingredients are:

• Overvaluation. Prices rise far ahead of what plausible future profits can support. In the late-1990s dot-com bubble, many internet companies with little revenue reached enormous valuations.
• Leverage. Investors who buy with borrowed money must sell when prices fall, to meet margin calls from their lenders. Forced selling pushes prices down further, triggering more margin calls.
• Panic and coordination. Once prices fall fast, the question shifts from "what is this company worth?" to "what will everyone else do?". Selling begets selling.
• Genuine shocks. Some crashes reflect a real change in the outlook: a pandemic, a banking crisis, a war.

Four crashes compared

Episode	Trigger and dynamics	What followed
1929	Speculative boom financed heavily on margin; forced selling as prices fell	Great Depression; new securities laws in 1933–34 and the creation of the SEC
19 Oct 1987	Dow fell 22.6% in one day amid portfolio-insurance selling and panic	Fed pledged liquidity on 20 October; circuit breakers introduced; US stocks regained pre-crash highs in under two years
2007–09	Losses on mortgage-backed securities spread through a highly leveraged banking system	Global recession; bank rescues; tighter capital rules
Feb–Mar 2020	COVID-19 lockdowns implied a sudden collapse in earnings; S&P 500 fell about 34% from 19 Feb to 23 Mar	Massive central bank and government support; S&P 500 closed at a record on 18 Aug 2020

The 1987 crash is a good case study in how policy responses evolved. According to the Federal Reserve's own history of the episode, Chairman Alan Greenspan announced on 20 October that the Fed stood ready to serve as "a source of liquidity", and behind the scenes encouraged banks to keep lending to securities firms. Regulators later introduced market-wide circuit breakers: under current rules, trading halts when the S&P 500 falls 7%, 13% or 20% in a day, giving participants time to pause rather than dump shares into a vacuum. Some economists argue that repeated rescues create moral hazard — the expectation that authorities will always step in can encourage the very risk-taking that causes crashes.

A crash in the stock market is not the same thing as a recession, though the two often travel together. Our explainer on what a recession is unpicks the difference.

Why markets have tended to rise over long periods

Despite repeated crashes, the major stock markets of the large developed economies have historically risen over long periods. The usual explanations are:

• Economic growth. As economies grow and firms become more productive, aggregate profits tend to grow too, and share prices follow profits over long horizons.
• Reinvestment and compounding. Companies reinvest profits, and investors who reinvest dividends earn returns on their returns, a process explained in compound interest explained.
• Inflation. Prices of goods rise over time, and so do companies' nominal revenues and profits, lifting nominal share prices even when real growth is modest.
• The equity risk premium. Because shareholders are paid last and bear more risk, they have historically demanded and received higher average returns than holders of government bonds — though not in every period, and not guaranteed in future.

Two important caveats. First, these are aggregate patterns for broad markets, not promises about any individual stock, many of which go to zero. Second, some national markets have suffered decades-long declines in real terms, and some were wiped out entirely by war or revolution. The long-run record of the US market, the most studied, is not a law of nature.

The stock market is not the economy

Stock prices reflect expectations about future corporate profits, discounted by interest rates and risk. Economic data such as GDP and unemployment describe the present or the recent past. The two can diverge sharply. In 2020 US share prices recovered within months while unemployment was still very high, partly because investors were looking ahead and partly because the largest listed companies were disproportionately in sectors that did well during lockdowns. Listed companies are also only part of any economy: millions of small businesses, government services and much of household activity never appear in an index.

Common behavioural pitfalls documented in research

Researchers in behavioural finance have documented patterns in how people trade:

• Overtrading. Every trade carries costs, including the bid-ask spread, so frequent trading tends to erode returns.
• Chasing performance. Money tends to flow into funds after strong performance and out after losses, so many investors end up buying after rises and selling after falls.
• Overconfidence. People tend to overrate their ability to pick winners or time the market.
• Loss aversion. Losses feel more painful than equivalent gains feel good, which can lead people to sell in panic or hold losers too long.

These are descriptions of observed tendencies, not advice. Anyone deciding how to manage their own savings should take personal advice from a licensed, independent professional.

Frequently asked questions

Q: What is the difference between the stock market and a stock exchange? A: A stock exchange is a specific venue, such as the New York Stock Exchange or Nasdaq, that runs an order book and lists companies. "The stock market" is the broader system of exchanges, alternative trading venues, brokers, clearing houses and regulators through which shares are issued and traded.

Q: Who sets stock prices? A: Nobody sets them directly. The price at any moment is where the most recent buyer and seller agreed. It changes whenever new orders arrive that are willing to trade at a different level, usually because expectations about the company, interest rates or risk have changed.

Q: Does a company get money when I buy its shares on an exchange? A: No. On the secondary market your money goes to the investor who sold the shares. Companies receive money only when they issue new shares, for example in an IPO or a later share offering.

Q: What happens to shareholders if a company goes bankrupt? A: Shareholders are paid last, after secured lenders, bondholders and other creditors. In most bankruptcies little or nothing is left for common shareholders, which is why equity is considered riskier than debt.

Q: What are circuit breakers? A: They are automatic trading pauses introduced after the 1987 crash. Under current US rules, market-wide trading halts when the S&P 500 falls 7%, 13% or 20% in a day, giving participants time to absorb information instead of selling into a disorderly market.

Q: Is the stock market the same as the economy? A: No. Share prices reflect expectations about future profits of listed companies, while economic statistics describe current output and employment across the whole economy. In 2020, US stocks recovered within months while the wider economy was still badly damaged.

Q: Who regulates the US stock market? A: The Securities and Exchange Commission, established in 1934, oversees exchanges, brokers and company disclosure. Its stated mission is protecting investors, maintaining fair, orderly and efficient markets, and facilitating capital formation. Self-regulatory organisations such as FINRA supervise brokers day to day.

The bottom line

A stock market is a machine for pooling and pricing ownership. Companies use it to raise capital; investors use it to buy and sell slices of those companies; and the constant stream of orders turns millions of individual forecasts into a single price. Those prices move when expectations about profits, interest rates and risk change, sometimes violently. Crashes come from stretched valuations, leverage and panic, and the modern safeguards — disclosure rules, circuit breakers, central bank liquidity, faster settlement — were mostly built in response to past crashes. Understanding the machinery does not tell anyone what to do with their money, but it does make the daily headlines a great deal less mysterious.

Sources

• US Securities and Exchange Commission — Mission
• Federal Reserve History — Stock Market Crash of 1987
• World Federation of Exchanges — Public markets post strong growth in 2025 despite geopolitical instability (27 Feb 2026)
• SEC Office of Investor Education and Advocacy — investor bulletin "New T+1 settlement cycle: what investors need to know" (2024)
• Royal Swedish Academy of Sciences — The Prize in Economic Sciences 2013
• S&P Dow Jones Indices — SPIVA U.S. Year-End 2024 scorecard

Related Reading

• What Are Stocks — and How Does the Stock Market Actually Work?
• How Does the US National Debt Work — and Should We Be Worried?
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• What Causes Inflation — and How Central Banks Fight It
• What Is Quantum Mechanics — and Why Does It Matter?