The S&P 500 is widely used as a benchmark for 500 large U.S. companies.
The index is maintained by S&P Dow Jones Indices and is designed to represent a broad segment of the U.S. large-cap stock market. It is weighted by free-float market capitalization, so companies with larger publicly available market values generally have greater influence on its movements.
The S&P 500 began in its modern form on March 4, 1957, after expanding from an earlier index of 90 stocks. Although its name refers to 500 companies, it can contain more than 500 stocks when some companies have multiple share classes represented.
The index is often confused with the Dow Jones Industrial Average, which contains 30 stocks and uses a price-weighting method. The Nasdaq Composite covers companies listed on the Nasdaq exchange, while the Russell 2000 focuses on smaller U.S. companies.