What is an index fund?

An index fund is a fund that does not try to pick winners. It holds the same investments, in the same proportions, as a published list — an index — and its performance follows that list wherever it goes.

First, what an index is

An index is a list with a rule. The rule decides what is on the list and how much each entry counts. A large-company index might hold the biggest listed companies in a market, weighted by their size, so a company worth twice as much as another occupies twice the space. The list is maintained by an index provider, not by the fund; when the rule says a company enters or leaves, every fund tracking that index adjusts to match.

What "tracking" means mechanically

A fund tracking an index buys what the list says, in the proportions the list says. When you buy a share of the fund, your money is spread across every entry at once. Nobody researches which entry will do best, because the design does not ask that question — the fund's job is to be the list, minus costs. The difference between the fund's return and the index's return is called tracking difference, and costs are its largest ordinary cause.

Why costs matter more than they look

A fund's yearly charge is taken from the fund itself, whether markets rise or fall. Consider purely hypothetical arithmetic: a charge of 1% per year removes roughly a tenth of your money over a decade before compounding effects, while a charge of 0.1% removes roughly one hundredth. Index funds are typically cheap to run because following a list requires no research department — which is why cost is the first line on any fund document worth reading.

What an index fund cannot do

It cannot fall less than its market: when the index drops, the fund drops with it, by design. It cannot exclude an entry you dislike if the rule includes it. And it cannot beat the list it follows. Anyone presenting an index fund as a way to outperform the market is describing something else.

Where to check the mechanics for any specific fund

Every regulated fund publishes a factsheet and a costs document naming the index it tracks, the charge it levies and the tracking difference it has produced. Those documents — not marketing pages — are where the mechanics above become checkable for the fund in front of you.