The S&P500 (.INX) tracks the top 500 large-cap US common stocks.
S&P's (Standard & Poor) 500 index is a more accurate barometer of the US equity market than that of the Dow Jones Industrial Average (DJIA) which has only 30 components, in that it's overall spread is much larger and less exposed to the individual performances of a few businesses. The S&P 500 index is updated every 15 seconds during market hours.
Like the Dow Jones, the S&P records large shifts in the market which reflect sentiment in the overall American economy as a whole, such as the late 90's Dot Com Boom and the subsequent crash, or even the 1930's Great Depression. There are several investment firms with an S&P tracker fund (examples are SPY or IVV). These funds work by investing in the same companies which compose the S&P 500 index.