What is an ETF?
Updated on August 24, 2026 · by Alex
An ETF is a fund that trades on an exchange as if it were a stock, but that holds dozens or hundreds of different assets inside. Buying one share is the same as buying a small slice of everything it holds.
Most ETFs are index funds: they track an index — the S&P 500, for example — by buying the same companies in the same proportions. Nobody is picking anything; they follow a rule.
With a single trade you get exposure to all five hundred companies in the index. It is the simplest way to diversify with little money.
Because it solves a hard problem in one move. Building a portfolio equivalent to the S&P 500 by hand takes capital and constant rebalancing work; the ETF does it for you, for an annual fee that on the largest ones is a few hundredths of a percent.
That fee — the expense ratio — is the number that weighs most over the long run. The gap between an ETF charging 0.03% and one charging 0.75% looks trivial and, compounded over thirty years, eats an enormous chunk of the final result.
Buying several ETFs thinking it adds diversification. Three different U.S. technology ETFs hold largely the same companies: you are not diversifying, you are doubling up. It is always worth looking at what is inside before adding another one.
The second mistake is not checking the ETF's trading volume. The big ones trade constantly and you can get in and out without trouble. Small or very niche ETFs can see little activity, which makes entering — and above all exiting — more expensive.
