What is an ETF? How exchange-traded funds work, costs and risks
An ETF (exchange-traded fund) is a fund that holds a basket of assets — often tracking an index — and trades on a stock exchange like a single share.
What an ETF is
An exchange-traded fund is an investment fund whose shares are listed on a stock exchange. Each share represents a slice of a portfolio — the 500 companies of the S&P 500, a basket of government bonds, physical gold, or a single sector such as technology. Unlike a traditional mutual fund, which is bought or sold once a day at a single price, an ETF can be traded throughout the session at the market price.
Most ETFs are passive: rather than trying to beat the market, they replicate an index as closely and as cheaply as possible. That is why they have become the basic building block of many long-term portfolios and regular savings plans.
How it works
An ETF's market price stays close to the value of its holdings thanks to a creation and redemption mechanism run by large dealers called authorised participants. Two numbers sum up its cost and accuracy:
Annual cost = amount invested × expense ratio (TER) Tracking difference = ETF return − index return
When the ETF trades above the value of its holdings (its net asset value), dealers create new shares and sell them; when it trades below, they redeem shares — which pulls the price back in line. Some ETFs hold the actual securities (physical replication); others use swaps to deliver the index return (synthetic replication). Some pay out dividends, while accumulating ETFs reinvest them automatically.
Costs — a worked example
Expense ratios on broad index ETFs are often well below 0.5% a year, while many actively managed funds charge 1% or more. The gap compounds. Invest 10,000 for 30 years at a gross 7% a year: at a 0.2% fee you end with about 72,000; at 1% you end with about 57,400 — around 14,500 less for the same market return.
Also check the bid–ask spread you pay on the exchange, which is tight for large, popular ETFs and wider for niche ones.
Risks and common mistakes
An ETF wrapper doesn't make its contents safe: a technology ETF falls with technology stocks, and a single-country ETF carries that country's currency and political risk. Leveraged and inverse ETFs reset their exposure daily, so over longer periods their returns can drift far from two or three times the index — they are built for short-term trading, not for buy-and-hold.
Another mistake is owning several ETFs that hold largely the same stocks, which adds cost without adding diversification. Check what is inside, the fee and the tracking difference before you buy.
Frequently asked questions
What is the difference between an ETF and a mutual fund?
Both pool investors' money. An ETF trades on an exchange throughout the day at market prices, usually tracks an index and tends to have lower fees; a traditional mutual fund is bought from the fund company at a once-a-day price and is more often actively managed.
Are ETFs safe?
An ETF is a structure, not a guarantee. A broad, diversified index ETF spreads risk widely, but its value still rises and falls with the market. Narrow, leveraged or inverse ETFs can be very risky.
What is an expense ratio (TER)?
The annual fee, as a percentage of your investment, that the fund deducts to cover its costs. A 0.2% expense ratio costs 20 a year on 10,000 invested; it is taken from the fund's value rather than billed separately.