Market capitalization (market cap) is the total dollar value of a company's outstanding shares of stock, calculated by multiplying the current stock price by the total number of shares outstanding. It's also used to measure the size of a stock exchange by summing the market caps of all listed companies. The NYSE has the world's largest exchange market cap at approximately $25 trillion, followed by NASDAQ at around $22 trillion. Market cap categories include mega-cap (over $200 billion), large-cap ($10-200 billion), mid-cap ($2-10 billion), small-cap ($300 million-$2 billion), and micro-cap (under $300 million). These categories help investors understand the risk profile and growth potential of different investments. Larger companies tend to be more stable but grow slower, while smaller companies offer higher growth potential with more volatility.