An ETF is a fund that owns a basket of assets — often hundreds or thousands of stocks or bonds — and sells shares in itself that trade on a stock exchange like any ordinary stock. Buy one share and you own a proportional slice of everything inside. Two things make it behave differently from a traditional mutual fund: it trades continuously through the day at a market price rather than once daily at a computed value, and it uses a creation-and-redemption mechanism with large institutions that keeps the market price tethered to the value of the holdings — the same mechanism that gives ETFs their tax efficiency.
Most explanations stop at “a basket that trades like a stock,” which leaves the interesting question unanswered: if the share price is set by supply and demand on an exchange, what stops it drifting away from what the fund actually owns? The answer is the part worth understanding.
1. The Basket
The fund holds real assets. A total-market equity ETF owns shares in thousands of companies; a bond ETF owns thousands of individual bonds; a gold ETF owns bullion. Divide the value of everything it owns, minus liabilities, by the number of shares outstanding, and you get net asset value — the honest per-share worth of the fund at that moment.
Most ETFs track an index, meaning the basket is chosen by a published rule rather than by a manager's judgement. That is why costs are low: following a rule is cheap. For how a fund's ongoing cost is deducted, see how an ETF expense ratio is charged.
2. The Exchange Listing
Shares of the fund are listed on an exchange and trade all day at whatever price buyers and sellers agree on. You can buy at 10:04am and sell at 2:15pm. A mutual fund, by contrast, transacts once per day at the NAV computed after the close.
An important consequence: when you buy an ETF share, your money usually goes to another investor, not to the fund. You are trading on the secondary market, and the fund's holdings are untouched. That is structurally different from a mutual fund, where your money flows in and the manager deploys it.
3. The Arbitrage Loop That Keeps the Price Honest
This is the mechanism that makes the whole thing work, and it involves institutions called authorized participants.
| Situation | What the authorized participant does | Effect on price |
|---|---|---|
| ETF trades above NAV | Buys the underlying basket, delivers it to the fund for newly created ETF shares, sells those shares at the higher market price | New supply pushes the price down toward NAV |
| ETF trades below NAV | Buys cheap ETF shares on the market, redeems them with the fund for the underlying basket, sells the basket | Demand pushes the price up toward NAV |
Both trades are profitable for the participant, which is the point: nobody enforces the tracking — the profit motive does it, continuously. It is why a large index ETF trades within pennies of its underlying value nearly all the time, and why the mechanism strains in thinly traded funds or during severe market stress, when assembling or valuing the basket becomes harder.
4. Why the Tax Treatment Is Different
Notice that redemptions are settled in kind — the fund hands over securities rather than selling them for cash. That detail produces the tax advantage.
When a mutual fund investor sells, the fund often must sell holdings to raise cash, and the resulting capital gain is distributed to everyone still in the fund, including people who did nothing. An ETF's in-kind exchange is generally not a taxable sale for the fund, so broad index ETFs frequently distribute no capital gains at all. Your tax bill stays largely under your control — you owe when you choose to sell. More in how ETFs are taxed and ETF vs mutual funds.
5. How It Actually Pays You
- Price appreciation. Holdings rise, NAV rises, share price follows; you realise it on sale.
- Distributions. The fund collects dividends and interest, subtracts expenses, and passes the rest through — usually quarterly for stock ETFs and monthly for bond ETFs. See do ETFs pay dividends.
A fund holding assets that generate no income — physical gold, say — offers only the first.
6. What It Costs, and What Can Go Wrong
The expense ratio is the headline cost, commonly 0.02%–0.09% for broad index funds, accrued daily inside NAV so it never appears as a charge. Sitting outside it: the bid-ask spread when you trade, any premium or discount to NAV, and the fund's own internal trading costs.
On risk, the honest framing is that “ETF” is a wrapper, not a risk level. A total-market index fund and a leveraged single-sector product are both ETFs and are not comparable. The dominant risk is simply that the assets inside can fall. Structure-specific risks are narrower: wider spreads and larger NAV deviations in thin or hard-to-value funds, especially under stress, and leveraged and inverse products that reset daily and are not built for long holds. A fund can also close — which returns your money rather than losing it, as covered in what happens when an ETF closes.
Sources
Fund structure, exchange trading, creation and redemption, and required prospectus disclosure: U.S. Securities and Exchange Commission, Mutual Funds and ETFs and Investor.gov on mutual funds and ETFs. Net asset value: Investor.gov, Net Asset Value. Typical expense-ratio ranges reflect fees observed across large US-listed ETFs and change over time.
Caveats
This describes US-regulated ETFs and is general information, not investment advice. Any specific fund's holdings, costs, distribution schedule, and risks are set out in its prospectus on the issuer's own site, which is the authoritative source when a third-party data provider disagrees.