How Does the Stock Market Work? A Plain-English Explanation

Written by Jim Norris · NorrisAI AlphaLens · Updated August 2026

Quick answer: The stock market is where buyers and sellers trade ownership stakes — shares — in public companies. Prices are set by supply and demand: when more people want to buy a stock than sell it, the price rises; when more want to sell, it falls. Over the long run, prices tend to follow business performance. In the short run, they follow sentiment, news, and momentum.

What a stock actually is

When a company goes public through an IPO, it sells ownership stakes to outside investors in the form of shares. Each share represents a fractional ownership claim on the company's assets and future earnings. If you own 100 shares of a company that has 1 million shares outstanding, you own 0.01% of that company.

That ownership entitles you to a proportional share of any dividends the company pays, and a vote on major corporate decisions. More practically, it means your investment rises and falls with the company's value as the market perceives it.

How prices are set

Stock prices are determined moment-to-moment by supply and demand. The market is essentially a continuous auction — buyers submit bids at prices they're willing to pay, sellers submit asks at prices they're willing to accept, and trades happen when the two sides agree.

What moves prices in the short run: earnings reports, economic data, interest rate decisions, management changes, analyst upgrades and downgrades, news events, and shifts in investor sentiment. In the short run, the market is essentially a voting machine — reflecting the collective mood of millions of participants.

What drives prices in the long run: the actual performance of the underlying business — revenue growth, profitability, competitive position, and cash generation. Over long periods, a stock's price tends to converge toward the value of the business behind it.

The two main exchanges

Most US stocks trade on either the New York Stock Exchange (NYSE) or NASDAQ. Both are electronic markets where orders are matched algorithmically. The exchange a stock lists on doesn't meaningfully affect how you trade it as an individual investor.

Market hours and after-hours trading

The main US market session runs 9:30 AM to 4:00 PM Eastern Time on weekdays. Pre-market trading runs from roughly 4:00 AM to 9:30 AM, and after-hours trading from 4:00 PM to 8:00 PM. Volume is much lower outside regular hours, which means prices can move more dramatically on less information.

What "the market" actually measures

When people say "the market went up today," they usually mean an index — the S&P 500 (500 large US companies), the Dow Jones Industrial Average (30 major companies), or the NASDAQ Composite (tech-heavy). These indices are weighted averages of their constituent stocks and serve as benchmarks for overall market performance.

The key insight for investors

The market's short-term price movements and a company's long-term business value are two different things. In the short run, a stock's price can diverge significantly from the underlying business value — driven by sentiment, news cycles, and momentum. That divergence is where opportunity lives for investors who do the fundamental research to know what a company is actually worth.

Professor Jeremy Siegel of the Wharton School of the University of Pennsylvania has documented that the long-term real rate of return from investing in stocks is remarkably durable — stocks have returned approximately 6–7% annually after inflation over more than two centuries of US market history. Short-term volatility is real; long-term returns have been consistent. — Stocks for the Long Run, McGraw-Hill

Benjamin Graham described the market as a voting machine in the short run and a weighing machine in the long run. Short-term prices reflect sentiment; long-term prices reflect business reality.

How AlphaLens helps you cut through market noise

AlphaLens focuses on the weighing machine side — evaluating the underlying business, not the daily price action. The 15 research frameworks cover everything from business fundamentals and competitive moats to valuation and earnings quality, giving you a clear picture of what a company is actually worth independent of what Mr. Market is pricing it at today.

Where to go deeper

For a detailed walkthrough of each research framework, see the complete guide to all 15 AlphaLens frameworks.

For definitions of investing terms, see the AlphaLens investing glossary.