What Is Personal FinanceAn index fund is a fund built to match the performance of a specific market index, such as a benchmark tracking a country's largest companies, instead of trying to beat the market by picking individual winners.
How it works
An index defines a rule. For example: "the largest companies listed on a given exchange." An index fund buys a basket of investments that mirrors that rule closely, so its performance moves with the index rather than one manager's stock picks.
Why fees stay low
An index fund follows a fixed, rules-based approach instead of paying analysts to research and pick investments. That keeps costs down compared to an actively managed fund. Fees show up as an annual percentage of your investment, called the expense ratio. A small difference in that percentage compounds into a large gap over decades.
Why average performance still works
An index fund does not try to beat the market. It tries to match it. Over long periods, a large share of actively managed funds have underperformed their benchmark index after fees. Past patterns do not guarantee future results, but this is why index funds get discussed as a simple, low-cost building block.
The link to diversification
An index fund holds many underlying investments at once, which spreads out the risk of any single company doing poorly. See what is diversification for more on why that matters.
Nothing here recommends a specific index fund, provider, or index. Fees, taxes, and the index tracked all vary and are worth checking before you invest.