The S&P 500 is a stock market index that tracks about 500 of the largest US public companies, weighted by market value.
It is maintained by S&P Dow Jones Indices and is the most common benchmark for 'the US stock market'. Members are chosen by a committee based on size, profitability and other rules.
Why it matters to you
Many retirement accounts hold S&P 500 index funds, so its moves directly affect a lot of people's savings.