One of the most comprehensive stock indexes in the U.S. since it´s composed for the 500 largest American companies and is used as an indicator not only of these companies´ performance but also as an indicator of the performance and health of the economy as a whole.
The S&P 500 actually means Standard and Poor 500. This index track the 5000 most commonly held stocks on the New York stock exchange or NASDAQ. The S&P 5000 measures the market capitalisation (total summed value) of all the shares of the 500 different companies.