When a financial news anchor says "the market was up today," they're summarizing billions of individual transactions into a single number. Understanding what those transactions actually are — mechanically, at the level of what computers and humans do — gives a clearer picture of what financial markets are and how they fail.
What a stock actually is
A share of stock represents a fractional ownership claim on a company's assets and future earnings. If a company has one million shares outstanding and you own one thousand of them, you own 0.1% of the company — entitled to 0.1% of any dividends paid and 0.1% of whatever's left if the company is liquidated.
Shares are created when companies issue them — primarily through an initial public offering (IPO) — and can be repurchased and retired. Trading on exchanges is secondary market activity: existing shares changing hands between investors, with the company receiving none of the proceeds.
The order book
At the core of any exchange is an order book — a continuously updated list of buy orders and sell orders with their prices and quantities.
A limit order specifies a price: "I will buy 100 shares of X at no more than $50.00." This order sits in the book until matched or cancelled.
A market order says "buy (or sell) immediately at whatever the best available price is." Market orders execute immediately against the best available limit orders in the book.
The bid is the highest price any buyer is currently offering. The ask is the lowest price any seller is currently accepting. The difference is the spread — typically fractions of a cent for liquid stocks, larger for thinly traded ones.
When you click "buy" in your brokerage app, your order travels to a broker, gets routed (often through several intermediaries) to an exchange or trading venue, and executes against the order book. For a liquid stock like Apple or Microsoft, this takes milliseconds.
Market makers
For markets to function, there need to be buyers and sellers available simultaneously. Market makers — firms that continuously post both buy and sell limit orders — provide this liquidity. They profit from the bid-ask spread: buying at the bid, selling at the ask, capturing the difference.
Market makers take inventory risk (they may hold positions they don't want while waiting to close them) and are compensated by the spread. Competition among market makers compresses spreads — in highly liquid markets, spreads are fractions of a cent. In illiquid markets, spreads can be significant.
This is why "liquidity" matters: a market with many market makers and many participants has tight spreads and allows large trades with minimal price impact. An illiquid market has wide spreads and large trades move prices significantly.
High-frequency trading
High-frequency trading (HFT) firms use computers to execute strategies in microseconds — faster than human reaction times by several orders of magnitude. They do this by co-locating their servers physically adjacent to exchange servers to minimize signal travel time.
HFT strategies include market making (faster and more efficient than humans), statistical arbitrage (exploiting price discrepancies between related securities), and latency arbitrage (trading on price information before slower participants receive it).
Whether HFT improves or harms markets is genuinely contested. Evidence suggests it has tightened spreads for retail investors (a benefit) while extracting value from institutional investors through latency advantages (a cost). The Flash Crash of 2010 — when the Dow dropped 9% in minutes before rapidly recovering — demonstrated that HFT strategies can amplify volatility in stress conditions.
Settlement
Execution and settlement are different steps. Most covered U.S. securities transactions settle on T+1: one business day after the trade date, not necessarily the next calendar day. Weekends, market holidays, and exceptions matter. Other markets can use different cycles; check the rules applicable to the security and market.
Between trade and settlement, your broker effectively extends credit. This is why brokers can have exposure to client positions even before cash and shares have moved — and why brokerage failures can cause problems even for clients whose trades were executed correctly.
What moves prices
In theory, prices reflect all available information about a stock's future cash flows discounted to present value (the efficient market hypothesis). In practice, prices reflect the combined beliefs and constraints of all current buyers and sellers, which includes rational analysis, momentum, sentiment, forced selling from leveraged investors, and index rebalancing.
Index funds — which buy and sell mechanically to track a benchmark — now represent a large fraction of the market. When a company is added to the S&P 500, index funds must buy it regardless of price. This mechanical buying pressure is one reason S&P 500 additions see price increases at announcement: it's not new information about the company's value, it's predictable forced buying.
The market is a mechanism for aggregating information and beliefs about value. It's a remarkably effective one. It's also a social construction that reflects human behavior as much as underlying economic reality — which is why it periodically overshoots, undershoots, and occasionally panics.