The stock market is the engine that connects people who want to invest money with companies that need money to grow. It works like an auction — buyers bid what they're willing to pay, and sellers ask for a price. When a buyer and seller agree on a price, a trade happens. This happens millions of times per second on modern electronic exchanges. The 'price' of a stock at any given moment is simply the last price at which a trade occurred.

Market hours in the US run from 9:30 AM to 4:00 PM Eastern Time, Monday through Friday. But there's also pre-market trading (4:00-9:30 AM) and after-hours trading (4:00-8:00 PM) for those who want to react to news outside regular hours. Volume and liquidity are lower in extended hours, which means bigger price swings. Most retail traders should stick to regular market hours until they understand how thinner markets work.

Behind every trade is a market maker — a firm that stands ready to buy or sell a stock at any time. Market makers provide liquidity (the ability to trade without moving the price too much) and they make money on the bid-ask spread: the small difference between the buying price and selling price. Firms like Citadel Securities and Virtu Financial handle a huge percentage of all US equity trades. Their role is essential — without market makers, you might not be able to sell when you want to.

The overall market is measured by indices like the S&P 500 (500 large US companies), the NASDAQ Composite (tech-heavy), and the Dow Jones Industrial Average (30 blue-chip stocks). When people say 'the market is up,' they usually mean the S&P 500 is up. These indices are weighted — bigger companies have a bigger impact on the index. That means Apple and Microsoft moving 1% moves the S&P more than a small company moving 5%.