Index funds that track the S&P 500 are available online from Vanguard, SPDR, Fidelity and other large brokerages. The S&P 500 is a listing of stocks, but it cannot be invested into directly. Investors must find a mutual fund which buys and sells stock to mirror the S&P 500.
The S&P 500, or Standard and Poor's 500, is a broad index of large capitalization stocks. It is widely considered an excellent barometer of the U.S. stock market, and most managed mutual funds are compared to the S&P 500 as a performance standard. Investing in the S&P 500 is considered by many advisors to be a good way to maintain a diverse and yet low-risk portfolio of equity investments.