An index fund is a type of mutual fund or exchange-traded fund (ETF) that holds all (or a representative sample) of the securities in a specific index, with the goal of matching the performance of that benchmark as closely as possible. The S&P 500 is perhaps the most well-known index, but there are indexes—and index funds—for nearly every market and investment strategy you can think of. You can buy index funds through your brokerage account or directly from an index-fund provider, such as BlackRock or Vanguard.

When you buy an index fund, you get a diversified selection of securities in one easy, low-cost investment. Some index funds provide exposure to thousands of securities in a single fund, which helps lower your overall risk through broad diversification. By investing in several index funds tracking different indexes you can build a portfolio that matches your desired asset allocation. For example, you might put 60% of your money in stock index funds and 40% in bond index funds.