What Is an ETF? How Exchange-Traded Funds Work

You type four letters into a brokerage app, pay for one share, and by the end of the afternoon you own a sliver of five hundred companies.

An exchange-traded fund, or ETF, makes that possible. It is a basket of investments that trades on a stock exchange as a single share, so one share buys a small slice of everything in the basket: 500 large US companies, a few thousand bonds, or gold sitting in a vault. For most households it is the cheapest way to own a diversified portfolio. The purchase genuinely does take about two minutes.

The two minutes are the easy part. What follows is everything underneath them.

One share, a claim on a pile

A fund pools money from many investors, buys securities with it, and issues shares that represent a claim on the pool, so diversification, meaning money spread across enough holdings that no single one can sink you, comes built into the arrangement.

Picture a fund holding the 500 companies in the S&P 500 in proportion to their market value, where market value is the share price multiplied by the number of shares in existence. You put in $1,000. That $1,000 now sits across 500 businesses. The biggest company takes the biggest slice.

Say one of those 500 goes bankrupt. You lose a fraction of a percent, you read about the collapse in the news, and your account balance carries on doing whatever the rest of the basket told it to do that week. Put the whole $1,000 into that one company and it is gone.

Day to day the fund does almost nothing. It holds what the index tells it to hold, adjusts when the index changes on the schedule the index provider publishes, and takes a fee out of the fund’s assets for the trouble. That fee is most of what separates a good ETF from an expensive one.

The trade that keeps the price honest

An ordinary fund has no market price to go wrong. An ETF does, so something has to stop the share price wandering away from the value of the holdings, and that something is a standing arrangement between the fund and a handful of large trading firms called authorized participants.

Suppose the price drifts above the holdings’ value. An authorized participant buys the underlying securities in the open market, hands them to the fund, receives newly issued ETF shares in exchange, and sells those shares on the exchange for more than the securities cost it. The selling pushes the price back down.

Now the price drifts below. The process runs backwards. The participant buys cheap ETF shares on the exchange, hands them to the fund, and takes delivery of the underlying securities, which are worth more than it paid. The buying pushes the price back up.

These swaps happen in blocks of 25,000 or 50,000 shares, called creation units. You will never see one. What you see is the result: for a large fund holding liquid assets, the market price sits within a few cents of the value of the holdings almost all of the time.

What the price is doing when it drifts

Net asset value, or NAV, is everything the fund owns minus what it owes, divided by the number of shares outstanding, and it is the honest per-share value of the basket, struck once a day after the US market closes at 4:00 p.m. Eastern time. The market price is whatever buyers and sellers agree on while the market is open. Above NAV, the fund trades at a premium. Below NAV, a discount.

On a large US stock fund the gap is a rounding error. You can ignore it.

A fund holding Japanese or European stocks is different. Those markets are shut while US investors are trading the fund, the ETF price moves on news the home market has not opened to yet, and a premium of 1% there is information you can use.

Bond funds during a panic are the interesting case. Individual bonds trade rarely, so the published NAV leans on estimated prices, while the ETF has real buyers and sellers setting a real price, and the discount you see may be the fresher of the two numbers.

Thinly traded niche funds are where the gap becomes your problem. Check the spread before you place the order.

The tax advantage, and the account where it is worthless

This section is about taxable brokerage accounts. Inside an IRA or a 401(k) none of this touches you, the fund’s tax plumbing is irrelevant to a sheltered account, and if that is where all your money sits you can skip to the next heading.

Investors leave a traditional mutual fund. It sells holdings to pay them out. Selling appreciated holdings creates a capital gain, and US rules make the fund distribute that gain to everyone still holding it, so you can receive a taxable capital gains distribution in a year when you sold nothing and your fund lost money. People find that as infuriating as it sounds.

An ETF mostly sidesteps this. When an authorized participant redeems shares, the fund hands over securities, and the tax code treats that kind of in-kind transfer as an exchange, so no taxable sale happens inside the fund. Managers also use those redemptions to send out their oldest, lowest-cost shares, which quietly walks the biggest embedded gains out of the portfolio.

Broad stock index ETFs therefore go years at a stretch without paying a capital gains distribution. You still owe tax on dividends each year, and you still owe capital gains tax on your own sales. The rest of the arithmetic sits in tax-efficient investing.

What holding one costs

The expense ratio is the annual fee, stated as a percentage of your money. A fund charging 0.03% takes $3 a year from a $10,000 position. One charging 0.75% takes $75. That sounds like a difference worth ignoring. It is not. Put $10,000 into each and let both grow at 7% a year for 30 years: the cheap fund compounds at 6.97% and reaches $75,485, the expensive one compounds at 6.25% and reaches $61,641. The gap is $13,844 on a $10,000 stake, and it never appeared on a statement. Run your own figures through the expense ratio calculator, or read the full treatment in expense ratios explained.

The bid ask spread is the gap between the highest price a buyer will pay and the lowest a seller will accept, and you cross it every time you trade. On a heavily traded ETF it can be one cent on a $500 share. On an obscure fund it can be half a percent, each way.

Tracking difference is how far the fund’s return lands from its index after fees, and a well run fund misses by hundredths of a percent a year.

Commissions on US-listed ETFs are zero at the major brokers. The spread is the whole cost.

Buying your first one

Open a brokerage account and fund it. Decide what you want to own, which for most people means one broad fund covering the whole US stock market or the S&P 500 itself, and tickers like VOO, SPY and VTI come up constantly in this conversation. They illustrate the category well enough, and which of them you pick matters far less than owning one at all.

Check its expense ratio and its assets. Enter a limit order for the number of shares you want, priced at or a few cents above the current ask. The shares appear in your account one business day later, under T+1 settlement.

Then set up a monthly contribution and stop looking at it. The step-by-step version lives in how to invest in stocks, and the case for contributing steadily, month after month, through good markets and bad, is worked through in dollar-cost averaging versus lump sum. How much of your money belongs in stock funds at all is a bigger question than which fund you buy, and it deserves the larger share of your thinking.

Where the wrapper stops protecting you

The structure itself is sound. People get hurt by what is inside it and by how they use it.

An ETF is only as diversified as its holdings. Forty companies from one industry make a concentrated bet wearing a diversified costume, which is the trap described in sector ETFs.

The ease of trading invites trading. A fund you can sell in one tap is a fund you can panic out of at the bottom, and mutual funds priced once a day gave their holders one accidental layer of protection from themselves.

Some products carrying the ETF name work very differently. Leveraged and inverse funds reset their exposure every evening and bleed value in choppy markets even when the index ends up where it started, as the day-by-day arithmetic in leveraged and inverse ETFs shows. Exchange-traded notes are unsecured bank debt. That bank’s solvency becomes part of your risk.

None of that is an argument against the wrapper. It is an argument for opening the holdings page before you buy anything, which takes about ninety seconds.

One job for this week

Pull up every fund you already own, write its expense ratio next to its balance, and total the dollars. Compare that total with what a broad index fund would have charged you for the same exposure, and if the two numbers land close together you are done and you do not need to change anything at all. The ETFs and index funds quiz will tell you what stuck.