The S&P 500 index tracks 500 large U.S. companies across 11 different industries and sectors to offer a picture of the health of the U.S. stock market and the broader economy. It represents the stock market's performance by reporting the risks and returns of the biggest companies. The stocks in the S&P 500 represent roughly 75% of all publicly traded stocks. Stock trading symbol “SPY” is an example of one of several Exchange Traded Funds (ETFs) which mimics the performance of the S&P 500 index..