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

How Does the Stock Market Work?

The stock market is where buyers and sellers come together to trade shares of publicly listed companies. The "market" isn't a physical location (though the New York Stock Exchange still has a famous trading floor) — it's a network of exchanges, electronic systems, and participants all operating under regulated rules.

At its core, the stock market answers one question every second of every trading day: what is this company worth right now?

The Basic Mechanics

Companies Go Public via IPO

A company raises capital by selling shares to the public for the first time through an Initial Public Offering (IPO). During the IPO, investment banks help the company set an initial price and sell shares to institutional investors first. After the IPO, those shares trade on an exchange like NYSE or Nasdaq — anyone with a brokerage account can buy and sell them in what's called the secondary market.

Exchanges Match Buyers and Sellers

Stock exchanges like the NYSE and Nasdaq are electronic marketplaces. When you place an order to buy 10 shares of Tesla, the exchange's matching engine pairs your order with someone who wants to sell 10 shares of Tesla at the same price.

This happens electronically in microseconds. Modern exchanges process millions of transactions per second. The NYSE alone handles over a billion shares on active days.

The Order Book and Bid-Ask Spread

Behind every stock price is an order book — a live, continuously updated list of all pending buy orders (bids) and sell orders (asks). The current stock price is the most recent price at which a buyer and seller agreed to transact.

• Bid price: What buyers are currently willing to pay
• Ask price: What sellers are currently willing to accept
• Spread: The gap between bid and ask — usually just 1 penny for large, liquid stocks like Apple, but wider for smaller or less-traded stocks

When you buy at market price, you pay the ask. When you sell at market price, you receive the bid. The spread is effectively a transaction cost you pay with every trade. Understanding this is key to understanding why limit orders often get you better prices than market orders.

How Prices Are Determined

Stock prices are determined purely by supply and demand at any given moment. If more people want to buy a stock than sell it, buyers must raise their bids to attract sellers — the price rises. If more want to sell, sellers must lower their asks — the price falls.

This continuous auction process runs every millisecond of the trading day. A single piece of news can instantly shift the balance: an earnings beat sends buyers flooding in; a product recall sends sellers rushing to exit. The price adjusts until buyers and sellers reach a new equilibrium.

Market Participants

The stock market has several distinct types of participants, each playing a different role:

Retail Investors

Individual people buying and selling through brokerage accounts. Retail investors account for roughly 20–25% of total market volume on an average day. They include everything from someone contributing $100/month to a Roth IRA, to active day traders executing dozens of trades daily.

Institutional Investors

Large organizations managing money on behalf of others. Hedge funds, pension funds, mutual funds, insurance companies, and endowments collectively account for the majority of market volume. They trade in sizes that can move stock prices — a pension fund buying $500 million of Apple stock will push the price up during its purchase.

Market Makers

Firms (like Citadel Securities, Virtu Financial) that provide liquidity by continuously posting both bid and ask prices for stocks throughout the trading day. They profit from the spread between what they pay for shares and what they sell them for. Market makers are required to maintain continuous quotes, ensuring there's always someone to buy from or sell to. Without market makers, trading would be slower and spreads would be much wider.

High-Frequency Traders (HFT)

Algorithmic trading firms that execute millions of trades per day in microseconds, exploiting tiny price discrepancies between exchanges. HFT firms have co-located servers next to exchange data centers to shave nanoseconds off order transmission. They provide liquidity but are controversial — critics argue they front-run retail orders.

Algorithmic Traders

Hedge funds and prop trading firms running automated strategies — statistical arbitrage, momentum, mean reversion — that execute based on quantitative signals. See what is AI trading for how these systems work.

How Your Order Flows from Click to Fill

Here's what actually happens when you press "Buy":

1. You submit an order in your brokerage app (e.g., "Buy 10 shares of AAPL at market price")
2. Your broker routes the order — Most retail brokers use payment for order flow (PFOF) to route orders to wholesale market makers like Citadel or Virtu. Others route directly to exchanges.
3. The market maker or exchange executes the trade — At the current ask price, your order is matched with a seller. For large brokers, this happens in under 100 milliseconds.
4. Trade confirmation is sent — Your broker shows the filled order with execution price.
5. Settlement occurs (T+1) — The exchange officially transfers ownership of shares to you and cash to the seller on the next business day. During this window, the trade is pending.

Exchanges vs. OTC Markets

Major exchanges like the NYSE and Nasdaq have strict listing requirements — minimum revenue, market cap, shareholder count. Companies must meet ongoing reporting requirements to stay listed. Exchange-listed stocks have the tightest spreads and most transparent pricing.

OTC (Over-the-Counter) markets are where smaller, unvetted companies trade. OTC stocks (often called "penny stocks") trade on platforms like OTC Markets rather than major exchanges. They have:

• Much wider bid-ask spreads (sometimes 10%+ spreads)
• Lower trading volumes and liquidity
• Less regulatory oversight
• Higher susceptibility to pump-and-dump schemes

Most beginner investors should stick to exchange-listed securities until they thoroughly understand OTC risks.

Market Hours

Regular session: NYSE and Nasdaq operate Monday–Friday, 9:30 AM – 4:00 PM Eastern Time (closed on federal holidays and early closes before major holidays).

Pre-market trading: 4:00 AM – 9:30 AM ET. Available at most brokers. Volume is low, spreads are wide, and price moves can be extreme. Most major earnings releases drop during this window, causing big pre-market moves.

After-hours trading: 4:00 PM – 8:00 PM ET. Similar characteristics to pre-market — low volume, wide spreads. Late afternoon earnings announcements move stocks significantly in this session.

Why pre/post market matters: If a company reports earnings at 4:30 PM after the close, the stock might gap up or down 15–20% before the regular session opens. If you want to react, you can trade in after-hours — but you'll face worse prices and less liquidity than during regular hours.

Circuit Breakers

To prevent panic-driven crashes, US exchanges have circuit breakers that halt trading when the S&P 500 drops sharply:

• Level 1 (7% drop): 15-minute trading halt
• Level 2 (13% drop): 15-minute trading halt
• Level 3 (20% drop): Trading halted for the remainder of the day

Individual stocks also have circuit breakers. If a stock moves more than 5–10% in a 5-minute window (depending on the stock's price tier), trading is paused for 5 minutes — a Limit Up-Limit Down (LULD) mechanism designed to prevent erroneous trades.

Circuit breakers were implemented after the 2010 Flash Crash, when the Dow briefly dropped 1,000 points in minutes due to a cascade of algorithmic orders.

What Drives Stock Prices?

Stock prices change because people's assessments of a company's value change. Key price drivers:

Earnings — Companies report earnings quarterly. If a company earns more than expected, the stock typically rises. Misses cause drops. The magnitude of moves depends on how far results deviate from consensus estimates.

Revenue and guidance — Future guidance (what management expects next quarter/year) often matters more than current results. A beat on earnings with a lowered forward outlook can still send a stock down.

News and events — New products, CEO changes, lawsuits, regulatory decisions, mergers and acquisitions — anything that affects future earnings potential moves prices.

Economic data — Interest rates, inflation (CPI), unemployment, and GDP growth all affect how investors value stocks. When interest rates rise, future cash flows are discounted more heavily, often pushing stock valuations down.

Supply and demand dynamics — Large institutional buy/sell programs, index rebalancing (when index funds must buy newly added stocks), and short squeezes can all drive short-term price action independent of fundamentals.

Sentiment and psychology — Markets are driven by human behavior. Fear and greed cause prices to overshoot in both directions — which is why bubbles and crashes happen even when fundamentals haven't changed.

Market Indexes

You'll often hear about "the market" going up or down. This refers to a market index:

• S&P 500 — Tracks 500 large US companies. Market-cap weighted, so Apple and Microsoft have outsized influence. The most widely used benchmark for US stock market performance.
• Dow Jones Industrial Average (DJIA) — Tracks just 30 major US companies. Price-weighted (a $500 stock has 5× the index impact of a $100 stock). Older and less representative than the S&P 500.
• Nasdaq Composite — Tracks all ~3,000 stocks on the Nasdaq exchange, heavily weighted toward technology companies.
• Russell 2000 — Tracks 2,000 small-cap US stocks. Often used as a gauge of small business health and economic optimism.

When someone says "the market was up 1% today," they usually mean the S&P 500 rose 1%.

How Stock Prices Reflect the Future

One crucial concept: stock prices don't reflect what a company earned last year — they reflect what investors expect the company to earn in the future. That's why a company can report record profits and still see its stock fall (if the profits came in below expectations), or why a money-losing startup can trade at a high valuation (if investors expect massive future profits).

This forward-looking nature is why news and surprises move prices so dramatically — markets are constantly updating their expectations. The stock market is sometimes called a "discounting mechanism" because it prices in anticipated future events before they happen.

Key Vocabulary

• Bull market — A period of rising stock prices, generally defined as a 20%+ rise from a recent low
• Bear market — A period of falling stock prices, generally defined as a 20%+ drop from a recent high
• Liquidity — How easily you can buy or sell without moving the price. Highly liquid stocks (Apple, Microsoft) can absorb huge orders without much price impact.
• Market cap — A company's total market value. Price per share × total shares outstanding. See the market cap glossary entry.
• Volatility — How much a stock's price fluctuates. High volatility means large price swings.
• Float — The number of shares actually available for public trading (total shares minus insider-held and restricted shares). Low-float stocks are more volatile.
• Short selling — Borrowing shares and selling them, hoping to buy them back cheaper later to profit from a price decline.

Related Reading

• What Is a Brokerage Account? — Your gateway to participating in markets
• Stocks vs. ETFs — The two most common things people buy in the stock market
• What Is ATR? — How traders measure volatility day-to-day

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