Capital Flow Watch

The S&P 500 is a free-float capitalization-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. (Wikipedia Feb 2010)