Standard and Poor's 500 Index covers 500 industrial, utility, transportation, and financial companies of the US markets (mostly NYSE issues). The index represents about 75% of NYSE market capitalization and 30% of NYSE issues. It is a capitalization-weighted index as calculated by Russell on a total return basis with dividends reinvested. Commonly referred to as the S&P 500.
For more information, see the U.S. Indexes section of the S&P web site.