How Stock Markets Work

Shares as Ownership Slices

When a company wants to raise money from the public, it can divide itself into millions of equal pieces called shares (also called stocks or equities). Buying one share makes you a part-owner of that company — a tiny fraction, but ownership nonetheless. If the company earns profits, shareholders may receive a portion as dividends; if the company grows more valuable, the shares tend to reflect that.

The total number of shares multiplied by the current share price equals the company's market capitalization — the market's live estimate of what the entire business is worth. Suppose a company has 100 million shares outstanding and each trades at $50; its market cap would be $5 billion (that is a hypothetical example). This single number is how investors, analysts, and indexes size up and compare companies.

Exchanges as Continuous Auctions

A stock exchange is, at its core, an organized marketplace that matches buyers with sellers. Every time you see a price tick on a shares page, it represents the most recent agreed price between a buyer who was willing to pay that amount and a seller who was willing to accept it. Nothing more complicated than that — the price is simply the last completed deal.

Modern exchanges run an electronic order book: a live, ranked list of every pending buy order (called a bid) and every pending sell order (called an ask or offer). The difference between the highest bid and the lowest ask is the bid-ask spread. A trade executes the moment a buyer's bid meets or exceeds a seller's ask. On heavily traded stocks, this happens thousands of times per second.

Liquidity — how easily you can buy or sell without moving the price — varies enormously across stocks. Large, well-known companies tend to have narrow spreads and deep order books. Smaller companies can have wide spreads and fewer participants, meaning even a modest order can shift the price noticeably. The full guide to market liquidity covers this in detail.

Market Hours, Opens and Closes

Exchanges operate during fixed windows. The New York Stock Exchange and Nasdaq, for example, run a regular session from 9:30 a.m. to 4:00 p.m. Eastern Time on business days, excluding public holidays. Outside those hours, some brokers offer pre-market and after-hours trading, but volume is typically much lower and spreads wider.

The open and close are not just bookends — they are the two highest-liquidity moments of the trading day, and they receive outsized attention. The opening price is set through a special opening auction that aggregates all overnight orders into a single starting price. The closing auction works similarly, and the resulting official closing price is the figure used by index providers, fund managers, and data vendors as the day's definitive price.

Earnings releases, economic data, and major news often drop outside regular hours, which is why pre-market moves can be sharp but may not fully carry into the regular session once full liquidity returns. The economic calendar lists scheduled events that traders typically watch around the open and close.

How Orders Work: Market vs. Limit

There are two fundamental order types every observer of markets encounters. A market order says "execute my trade immediately at whatever the current best price is" — it guarantees execution but not the exact price, which matters when a stock is moving fast or thinly traded. A limit order says "execute my trade only at this specific price or better" — it gives price control but no guarantee the order fills, because the market may never reach that price. Most of the orders sitting in an exchange's order book at any moment are limit orders; market orders arrive and consume them.

What Actually Sets a Share Price, Minute to Minute

In the very short term, a share price moves because the balance of buy and sell orders shifts. If more people want to buy than sell at the current price, buyers must offer a little more to entice sellers, and the price ticks up. If sellers outnumber buyers, prices drift down until buyers are attracted. This push-and-pull happens continuously throughout the session.

What drives those order flows? Almost everything. Company-specific news — a surprise profit, a product recall, a leadership change — can trigger a burst of buying or selling in a single name. Broader forces matter too: inflation data, central bank announcements, and shifts in risk-on / risk-off sentiment can move entire markets at once. During earnings season, individual stocks can gap sharply when reported results diverge from what analysts expected.

Over longer periods, prices tend to track fundamentals: revenue growth, profitability, debt levels, and the competitive landscape. The price-to-earnings ratio — a stock's price divided by its earnings per share — is one of the most-watched ways to gauge whether a price looks high or low relative to what the company actually earns. But on any given minute, price is simply what buyers and sellers agree it is right now.

From Single Stocks to Indexes

A stock index is a calculated number that summarizes the price performance of a defined group of stocks. Rather than watching hundreds of individual shares, market participants use indexes as a single thermometer for an entire market or sector. The S&P 500, for instance, tracks 500 large U.S. companies; the Nasdaq-100 focuses on the largest non-financial companies listed on the Nasdaq.

How those individual prices are combined depends on the index's weighting method. Most major indexes today are market-cap weighted: larger companies count for more. If a hypothetical company makes up 5% of an index and its stock rises 10%, the index gets a roughly 0.5-percentage-point boost from that one stock alone. A handful of very large companies can therefore have an outsized influence on what the index does on any given day. The guide to price-weighted vs. cap-weighted indexes explains the mechanics fully.

You can follow live index levels and the individual shares that make them up on the stocks and shares pages. Percentage-change columns — day, week, month, year-to-date, and year-on-year — are usually more informative than raw index levels, because a point move means something very different at different absolute levels. The guide to reading percentage moves explains why those columns matter more than the headline number.

Putting It All Together

Stock markets are, at their heart, machines for price discovery — they aggregate the opinions, information, and risk appetite of millions of participants into a single number, updated in real time. Shares represent ownership; exchanges provide the auction structure; order books match buyers and sellers; hours create predictable liquidity events; and indexes roll thousands of individual auctions into a single, trackable measure of the whole.

Understanding these mechanics is the foundation for making sense of everything else: why a market-wide drop can hit a small stock harder than a large one, why prices can lurch violently on low-volume mornings, and why the closing price carries special weight in financial data. The complete guide to stock indexes is a natural next stop for building on what you have just read.