What Is an Index Fund? (2026)

If you read anything about investing, you’ll quickly run into “index funds”. They’re the foundation of how most sensible long-term investors put their money to work, and they’re simpler than they sound. Here’s what they are and why they matter.

First, what’s an index?

An index is just a list that measures a chunk of the market. The S&P 500 tracks 500 large US companies; the FTSE All-World tracks thousands of companies across the globe. An index is a yardstick: when people say “the market went up 2%”, they usually mean an index did.

What an index fund does

An index fund is a fund that simply tries to match an index by holding all (or most) of the same investments in the same proportions. If the index holds 500 companies, the fund holds those 500 companies. When the index rises or falls, the fund does roughly the same.

This is called passive investing, because the fund isn’t trying to be clever or beat the market; it just tracks it. That sounds unambitious, but it’s exactly why it works so well.

Why index funds win

• Low cost. Because there’s no expensive team trying to pick winners, index funds are very cheap to run, often a fraction of the cost of a managed fund. Low fees mean more of the return stays with you, and costs compound over time.
• Instant diversification. One global index fund spreads your money across thousands of companies, so no single failure can sink you.
• They beat most of the alternatives. Over the long run, most actively managed funds fail to beat their index after costs. See passive vs active investing.
• Simplicity. One or two index funds can be a complete portfolio, with nothing to research or tinker with.

Index fund or ETF?

You’ll see index funds in two wrappers: traditional index mutual funds and index ETFs (which trade on a stock exchange). They do the same job; the differences are mostly practical. For European investors, broad UCITS ETFs are the common choice. See ETF vs index fund for the distinction.

How to use one

For most people, a single broad global index fund (an all-world ETF) is an excellent core holding: cheap, diversified and hands-off. Buy it regularly, hold it for years, and you’ve done the hard part. For how to put a couple together, see how to build a simple portfolio.

The bottom line

An index fund tracks a slice of the market cheaply and automatically, giving you broad diversification for very little cost. It’s the boring, proven core of most good long-term portfolios. Pick a broad, low-cost one, hold it through ups and downs, and let it compound. Compare brokers to hold it in on Brokerlens.

Educational information, not personal advice. All investing carries risk, so consider your own situation.