The stock market is where shares of publicly traded companies are bought and sold. Prices rise and fall continuously based on what buyers and sellers agree a share is worth at any given moment. Understanding how markets actually work — exchanges, indices, what drives prices — makes it possible to invest with realistic expectations rather than reacting to headlines.
Stock Exchanges and How Trading Works
A stock exchange is a marketplace — either a physical location or an electronic system — where buyers and sellers transact in shares of publicly traded companies. The two largest U.S. exchanges are the New York Stock Exchange (NYSE) and Nasdaq. When you place an order through a brokerage, the trade is routed to one of these exchanges or to an alternative trading system that operates under similar rules.
Market Orders and Limit Orders
A market order buys or sells shares immediately at whatever the current price is. A limit order buys or sells only at a price you specify — or better. Market orders are faster and nearly always execute, but you may pay slightly more or receive slightly less than the price you saw when you clicked. Limit orders give you price control but may not execute at all if the stock never reaches your target. For most investors making routine purchases in liquid stocks or ETFs, market orders during regular trading hours work fine. Limit orders make more sense for thinly traded stocks or when the exact price matters significantly.
Bid, Ask, and the Spread
At any moment, there is a bid price — the highest price a buyer is currently willing to pay — and an ask price — the lowest price a seller is currently willing to accept. The difference between the two is the spread. For major stocks and ETFs, spreads are often a fraction of a cent. For less actively traded securities, spreads can be much wider, which effectively adds a hidden cost to every transaction. When you buy at market, you pay the ask price; when you sell at market, you receive the bid price.
Market Hours and Pre/After-Hours Trading
The regular U.S. stock market trading session runs from 9:30 a.m. to 4:00 p.m. Eastern time on weekdays, excluding market holidays. Pre-market trading (4:00 to 9:30 a.m.) and after-hours trading (4:00 to 8:00 p.m.) are available through most brokerages, but volume is much lower and spreads are wider. Significant news — earnings reports, economic data releases, major announcements — often comes out before or after regular hours, which is why you sometimes see a stock’s after-hours price diverge sharply from its 4:00 p.m. close.
Watch: what is the stock market and how it works
Stock Market Indices: What They Measure
Stock market indices track the collective performance of a group of stocks. When the news reports that “the market was up today,” it is almost always referring to one of three major indices. Understanding what each one actually measures helps put market movements in context.
The S&P 500
The Standard & Poor’s 500 is widely considered the most representative measure of the U.S. stock market. It tracks 500 large U.S. companies selected by a committee, weighted by market capitalization — meaning larger companies have more influence on the index’s movement. Because it includes companies across all major industries and represents roughly 80% of the total U.S. stock market value, the S&P 500 is the benchmark most professional investors measure their performance against. When people refer to “the market” returning roughly 10% annually over long periods, they are typically referring to the S&P 500.
The Dow Jones Industrial Average
The Dow Jones Industrial Average — often just “the Dow” — tracks 30 large, well-established U.S. companies. It is the oldest and most widely cited stock index in the world, but it is less representative than the S&P 500 because it includes only 30 companies and uses a price-weighted methodology (meaning a higher-priced stock has more influence than a lower-priced one, regardless of company size). A 200-point move in the Dow sounds dramatic but means less than it once did — the index has grown substantially over decades, so the same point move represents a smaller percentage change than it used to.
The Nasdaq Composite
The Nasdaq Composite tracks all stocks listed on the Nasdaq exchange — over 3,000 companies — with a heavy concentration in technology. Because technology companies have driven a large share of stock market gains over the past two decades, the Nasdaq has often outperformed the S&P 500 in bull markets and fallen harder in downturns. The Nasdaq-100, a subset tracking the 100 largest non-financial Nasdaq stocks, is what most Nasdaq-focused index funds track. Investors who want broad diversification typically favor S&P 500 funds over Nasdaq-focused funds, given the latter’s technology concentration.
What Moves Stock Prices
Stock prices change because investors continuously update their expectations about a company’s future earnings and about the broader economic environment in which it operates. No single factor drives prices — it is always a combination of company-specific and macro influences.
Earnings and Company Performance
The most direct driver of a stock’s price is the company’s earnings — how much profit it generates relative to what investors expected. Companies report earnings quarterly, and the market’s reaction depends less on whether earnings are good or bad in absolute terms than on whether they are better or worse than what analysts had forecast. A company can report record profits and see its stock fall if the numbers missed expectations. Conversely, a company losing money can see its stock rise if losses were smaller than feared. Over long periods, earnings growth and stock price growth tend to converge.
Interest Rates and the Economy
When the Federal Reserve raises interest rates, it makes borrowing more expensive and makes bonds and savings accounts more attractive relative to stocks. This tends to put downward pressure on stock valuations — particularly growth stocks, whose value depends heavily on future earnings discounted back to today. When rates fall, the opposite effect applies. Broader economic data — GDP growth, unemployment, inflation — also moves markets because investors are constantly adjusting their expectations about future corporate profits based on the economic environment those companies will operate in.
Sentiment, News, and Short-Term Noise
In the short run, stock prices are heavily influenced by investor sentiment — fear and optimism that may not be directly tied to underlying business fundamentals. A geopolitical shock, a surprising economic report, or a single tweet can move markets in ways that seem disconnected from business reality. This short-term noise is why most financial research consistently shows that individual investors who try to time the market based on news tend to underperform those who hold index funds and do not trade in response to headlines. Over years and decades, fundamentals reassert themselves; over days and weeks, they often don’t.
Who This Page Is For
- Anyone who owns stock funds or a 401(k) and wants to understand what they are actually invested in
- People who feel anxious about market downturns and want a clearer mental model of how prices move
- Those who are new to investing and want to understand the mechanics before buying anything
- Anyone who has heard financial news references to the Dow or S&P 500 and was not sure what they actually measure
- People who want to understand the difference between investing and trading before deciding their approach
What to Do Next
- If you do not yet have a brokerage account, look at major providers like Fidelity, Vanguard, or Schwab — all offer commission-free trades and no minimum balance requirements for basic accounts
- Read the Index Funds and ETFs page to understand the most practical way most individuals participate in the stock market
- Review your 401(k) or IRA holdings to see which indices your current funds track — most target-date funds hold a combination of S&P 500, international, and bond index funds
- If you are unsure what mix of stocks and bonds is right for your situation, read the Asset Allocation page before making changes to your portfolio
- Avoid making investment decisions based on short-term market news — the evidence on market timing is consistent and discouraging for individual investors
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