Index funds are mutual funds that mimic the performance of a market index, like the S&P 500 or the DJI.
Index funds are a form of passive investment. Active investment consist in picking stocks or bonds to try to beat the market. Active investment assumes that market failures can be exploited to archive a larger performance. Active investment has a higher cost than passive investment, because it requires research. That is why active managed funds have much larger expenses than index funds.
Some of the biggest US index funds are: