The S&P 500 is a market free float-weighted index published since 1957 of the prices of 500 large-cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock market companies; the NYSE Euronext and the NASDAQ OMX.

After the Dow Jones Industrial Average, the S&P 500 is the most widely followed index of large-cap American stocks. It is considered a bellwether for the American economy, and is included in the Index of Leading Indicators. Some mutual funds, exchange traded funds, and other funds such as pension funds, are designed to track the performance of the S&P 500 index. Hundreds of billions of US dollars have been invested in this fashion.

The index is the best known of the many indices owned and maintained by Standard & Poor's, a division of McGraw-Hill. S&P 500 refers not only to the index, but also to the 500 companies that have their common stock included in the index. The ticker symbol for the S&P 500 index varies. Some examples of the symbol are GSPC, .INX, and $SPX. The stocks included in the S&P 500 index are also part of the broader S&P 1500 and S&P Global 1200 stock market indices.

Interesting links:

http://en.wikipedia.org/wiki/S&P_500

http://www2.standardandpoors.com

http://www.google.com/finance?q=INDEXSP:.INX