From Investopia.com….
The S&P 500 Index or the Standard & Poor’s 500 Index is a market-capitalization-weighted index of 500 of the largest publicly traded companies in the U.S.. It is not an exact list of the top 500 U.S. companies by market capitalization. because there are other criteria to be included in the index. The index is widely regarded as the best gauge of large-cap U.S. equities. Other common U.S. stock market benchmarks include the Dow Jones Industrial Average or Dow 30 and the Russell 2000 Index, which represents the small-cap index.
