What is an "index fund"?
An index fund is a mutual fund that buys securities that represent some index. As an example, if we were to take the S and P500 index, an index fund that was representing that index would buy those 500 securities in the same relative weighting as the index has those securities. It's really just implementing that index with real stocks. One of the good advantages of an index fund is they tend to have really low expense ratios and they have very low turnover, which are key factors when looking at the quality and the long term expected returns of a mutual fund.