How the Stock Market Works

Every day, hundreds of millions of people collectively decide the “correct” price of Apple, Amazon, and thousands of other companies. Most of them have never read a financial statement. They’re reacting to news, following trends, trusting gut instinct, and sometimes just copying what other people are doing. And yet, somehow, the prices that emerge from all this chaos are remarkably hard to consistently beat, even for professional investors with research teams and billions of dollars at stake. That’s the strange genius of markets.

The short answer

A stock is a tiny slice of ownership in a company. When you buy a share, you become a part-owner, entitled to a slice of the company’s profits (if they pay dividends) and a vote in major decisions. The stock market is simply a marketplace where these ownership slices get bought and sold, with prices moving up and down based on what people think those future profits are worth.

The full picture

What a stock actually represents

When you buy one share of Apple, you own roughly 0.000001% of the company. That’s not a typo. Apple has about 15 billion shares outstanding, so your single share is a tiny fraction. But that fraction comes with real rights: you get to vote on who sits on the board, and if Apple ever gets sold or liquidated, you get your proportional cut.

The price of a stock isn’t about what the company is worth today. It’s about what investors think the company will be worth in the future. If everyone believes Apple will make massive profits in 2027, the stock price climbs today to reflect those expected future profits. This is why stocks often seem to move on news about things that haven’t happened yet.

How exchanges work

The New York Stock Exchange (NYSE) and Nasdaq aren’t physical places with traders yelling in a pit anymore. They’re mostly electronic networks connecting buyers and sellers. Nasdaq launched in 1971 as the world’s first electronic stock market, now handles the majority of US stock volume (the SIFMA Capital Markets Fact Book tracks these figures annually), with the NYSE accounting for a significant share of large-cap trading. When you click “buy” on your brokerage app, your order travels through several layers of intermediaries before matching with a seller.

Here’s the key thing: exchanges don’t set prices. They just match buyers and sellers. The price is whatever someone is willing to pay and another is willing to accept.

Market makers and why liquidity matters

Every stock you can trade has designated market makers, firms legally obligated to step in and provide liquidity. If you want to sell 100 shares of Tesla but no buyer exists at that moment, a market maker will buy those shares from you. They make money on the bid-ask spread, the difference between the price they pay you and the price they charge the next buyer.

The bid is the highest price someone is willing to pay. The ask is the lowest price someone will accept. For heavily traded stocks like Apple or Microsoft, this spread might be just a penny. For tiny stocks trading only a few thousand shares per day, the spread can be 5% or more. That spread is a hidden cost every time you trade.

How prices actually move

Prices move because of the constant tug-of-war between buyers and sellers, but it’s not purely rational. Every price reflects aggregated expectations about the future, and those expectations get updated constantly as new information arrives.

When a company reports earnings that beat expectations, the stock typically rises because those future profit estimates just got revised upward. When the Federal Reserve announces interest rate changes, stocks move because interest rates affect how future profits are valued today. When a CEO gets arrested, the stock plummets because someone’s assessment of the company’s future just got dramatically worse.

But here’s where it gets weird: sometimes stocks move on completely unrelated things. During the 2023 banking crisis, dozens of unrelated stocks dropped simply because fear was in the air. The Dow Jones Industrial Average, which tracks just 30 large companies, dropped 900 points in a single day not because those 30 companies did anything wrong, but because panic spread.

Market indices: the shorthand for “the market”

When you hear “the market is up,” it almost always refers to an index, a mathematical average of a basket of stocks. The S&P 500 includes 500 of the largest US companies, weighted by market value. If Apple is the biggest company, it has the most influence on the index’s movement. The Dow Jones Industrial Average is older and weirder, just 30 companies weighted by stock price rather than market cap, which means a $10 stock moving $1 matters more than a $500 stock moving $1.

Index funds track these baskets. When you buy a fund that tracks the S&P 500, you’re essentially buying a tiny slice of 500 companies simultaneously. This is why index funds have become the default recommendation for most investors: they’re diversified, cheap, and historically beat most actively managed funds.

Bull and bear markets

A bull market is defined as a period when stocks are generally rising, typically 20% or more above recent lows. A bear market is the opposite, 20% or more below recent highs. These terms come from how each animal attacks: a bull thrusts upward, a bear swipes downward.

Bull markets tend to last years and rise gradually. The bull market that began in March 2009 (when the S&P 500 closed at a cycle low of 676.53 on March 9th, the closing nadir after the 2008 financial crisis) continued until early 2020, with the index reaching 3,386 by February 19, 2020. Bear markets are shorter but more violent. The Covid crash in March 2020 wiped 34% off the S&P 500 in just 33 days before recovering completely by August, one of the fastest crash-and-recovery cycles in market history, per Investopedia’s crash timeline.

Understanding this cycle matters because it explains why timing the market is so hard. Nobody knows when a bull market will end until it’s already over.

Institutional versus retail investors

There’s also a darker side of markets worth understanding: short selling, where investors borrow shares and bet on price declines, adding liquidity and price-discovery to the system.

When people talk about “the market,” they often imagine millions of individual investors buying and selling. That’s not quite right. Historically, roughly 70–80% of US stock trading volume came from institutional investors, massive entities like pension funds, hedge funds, mutual funds, and insurance companies. They trade in blocks of millions of shares, often using sophisticated algorithms.

Retail investors, individual people trading through brokerage apps, make up the remainder. According to SIFMA’s annual market-structure analysis, retail investors now represent approximately 20–30% of total equity volumes (up from around 10% historically), partly driven by the rise of zero-commission trading apps. But they remain relatively small players in terms of overall volume and market impact.

Here’s the uncomfortable truth: institutions have better information, better technology, and more resources. They can hire teams of analysts to study companies. Retail investors compete against this advantage, which is why strategies like buying index funds and holding for years tend to work better than trying to outsmart the professionals.

Why markets sometimes seem irrational

If prices reflect rational assessments of future value, why do bubbles form? Why do stocks crash on seemingly good news? Why do companies with zero profits trade at astronomical valuations?

Part of the answer is that humans aren’t perfectly rational. We get caught up in momentum, we fear missing out, we panic when others panic. When everyone is buying, the fear of being left behind overrides careful analysis. When everyone is selling, the fear of total collapse does the same.

Another part is that “rational” doesn’t mean “correct.” Two investors can look at the same data and reach completely different conclusions about the future. One might think Apple’s valuation is ridiculous, the other might think it’s still too cheap. Both have rational arguments. The market price is simply where their opposing views meet.

The 2008 financial crisis showed this clearly. Mortgage-backed securities, once considered safe, turned out to be ticking time bombs. The people who saw it coming and shorted the market made billions. The people who didn’t, lost everything. Both groups had access to the same information. Markets don’t reflect truth. They reflect aggregated human judgment, which is powerful but far from perfect.

Orders: market versus limit

When you buy a stock, you have two basic choices. A market order simply executes at whatever the current price is. If the ask price is $150.00, you pay $150.00. This guarantees execution but not price.

A limit order lets you specify the maximum you’ll pay (for buying) or minimum you’ll accept (for selling). If you place a limit order to buy at $145, your order only executes if someone is willing to sell at $145 or lower. This guarantees price but not execution. Your order might sit there forever if the stock never drops to your price.

Most casual investors use market orders because they’re simple. Active traders use limit orders to control their entry and exit prices precisely, accepting that some orders simply won’t fill.

High-frequency trading: the layer most investors don’t see

There’s a version of the stock market that operates at a scale most people never directly interact with, but which shapes every price they see.

High-frequency trading (HFT) firms use specialized computers and co-location agreements (literally placing their servers inside or adjacent to exchange data centers) to execute trades in microseconds. A typical HFT firm might make millions of trades per day, holding positions for milliseconds before closing them.

The business model is based on tiny margins executed at enormous volume. An HFT firm might profit half a cent per share, but across billions of shares traded per day, those fractions add up to hundreds of millions per year. They profit by being faster than everyone else: seeing order flow before others can react to it, or exploiting momentary discrepancies in prices between different exchanges.

The controversial practice is called front-running. If an HFT firm can see a large order coming in, say, a mutual fund buying a million shares of Apple, and execute its own purchase a millisecond before that order hits the market, it can sell those shares at a slightly higher price to the mutual fund. Technically legal in most forms. Ethically murky.

The net effect on ordinary investors is debated. HFT firms argue they provide liquidity and narrow bid-ask spreads, making markets cheaper for everyone. Critics argue they’re extracting value from slower participants without adding real economic benefit. What’s clear is that the market you see on your brokerage app is a simplified view of a system that’s operating at speeds and scales that no human can directly perceive, let alone compete with.

Why it matters

Understanding how the stock market works isn’t just academic. It affects your financial future. If you think stocks are magic tickets to wealth, you’ll make reckless decisions. If you understand that prices reflect collective judgment about the future, you’ll be more patient during crashes and more skeptical during booms.

The average individual investor underperforms the market by about 1.5% per year, mostly due to bad timing, a pattern consistently documented in DALBAR’s annual Quantitative Analysis of Investor Behavior that tracks investor behavior against benchmark indices. Understanding how the market actually works is your first defense against these mistakes.

You don’t need to pick stocks. You don’t need to time crashes. You just need to understand that ownership in companies is what you’re buying, that prices reflect expectations about the future, and that patience is the real edge.

Common misconceptions

Stocks are like lottery tickets. Each share represents real ownership in a company with assets, employees, revenue, and liabilities. The price is just what the market thinks that ownership is worth at this moment.

The market owes you money. Stocks go down as often as they go up. There’s no rule saying prices must recover. Some companies go to zero and stay there. Diversification is protection against the reality that some bets fail.

News moves markets the way you think. When earnings are announced, the stock price often moves before you even read the news. By the time something is public knowledge, it is already priced in. What moves markets is surprise, the gap between expectations and reality.

Professional investors always beat the market. They often don’t. After fees, most professional investors underperform simple index funds over time. The market is hard to beat consistently.