Market capitalization (market cap) is the total value of a company's shares: the share price multiplied by the number of shares outstanding.
Companies are often grouped as large-cap, mid-cap or small-cap. The S&P 500 weights its members by market cap, so the biggest companies have the most influence.
Why it matters to you
Market cap tells you how big a company is in the stock market's eyes, which helps compare companies with very different share prices.