What Is an Index Fund
The plain answer
Section titled “The plain answer”An index fund is a mutual fund or ETF that tries to match a published list of investments, such as a total stock market index. It is not trying to outsmart that list. It is trying to own it at a low cost.
You give up the chance of large outperformance. You also give up the common result of active funds, which is lagging the index after fees.
How it actually works
Section titled “How it actually works”An index is a rule for weighting a set of securities. The fund buys those holdings and rebalances when the index changes. Tracking error is how far the fund’s result drifts from the index, often because of fees and small timing differences.
Because the fund is not paying a large research staff to pick winners, the expense ratio can be very low. That fee difference compounds over years.
Index funds still fall when the market they track falls. Matching the market includes matching the declines.
What this means for you
Section titled “What this means for you”If you want a plan you can keep through busy years, a few broad index funds can be the whole portfolio. The work is saving, staying allocated, and leaving the funds alone.
Read what the index includes. A “stock” index might be only large U.S. companies, or it might be the world. The name on the ticker is not a substitute for the holdings list.
Common mistakes
Section titled “Common mistakes”Buying several overlapping index funds and thinking that is extra diversification.
Switching indexes after a bad year.
Assuming an index fund cannot lose money.
Related pages
Section titled “Related pages”Educational content, not personalized financial, tax, or legal advice. No affiliate relationships. Figures are for tax year 2026 and change annually.Read the full disclaimer.