The S&P 500 is an index made up of 500 of the largest companies in the U.S.

Officially called the Standard & Poor's 500 and created by the company Standard & Poor's, the S&P 500 is a market-capitalization-weighted index. This means that the individual components of the index — the stocks — that have a higher market capitalization are weighted more heavily in the index.

The S&P 500 certainly shouldn't make up your entire stock portfolio, since it's important for diversification purposes to also have smaller and international companies that may help your portfolio weather market volatility. And if you do want to have a say over which companies your money is going towards, you don't have that control when you invest in an index fund.

Investing in company stocks has been profitable for a long time. Read more on the RoboMarkets Blog for information on how to invest in s&p to get the most out of your investment.