The Standard & Poor's 500 (S&P 500) index is a composite of 500 United States' companies. Standard & Poor's, a financial analysis firm, select and maintain the companies in the index composite. Along with the Dow Jones Industrial Average, the Standard & Poor's 500 is a barometer of the U.S. stock market and business climate.
An investor looks at the DJIA close and wants to know how a broader selection of companies traded that day. She reviews the S&P 500 to make this comparison.