In plain English

An index fund holds the same investments as an index, in the same proportions. There is no manager picking stocks, which keeps costs low. An index fund can be a mutual fund or an ETF.

Example

An S&P 500 index fund holds the roughly 500 companies of the index. If the index rises 10%, the fund aims to rise about 10% before fees.

Why it matters

Low costs and broad diversification are the main attractions. Dividends from the companies in the index are collected by the fund and paid to you.

Common mistakes

  • Thinking all index funds are the same. They follow different indexes with different risks.
  • Ignoring the expense ratio.