What Is an ETF?

An ETF is a pooled investment fund. When you buy one share, you own a small piece of everything the fund holds. A total US stock market ETF might hold more than 3,000 companies; an S&P 500 ETF holds the roughly 500 largest US firms in the same proportions as the index. Instead of researching and buying each company yourself, you buy one ticker and get the whole basket in a single trade.

The "exchange-traded" part is what sets it apart from an older structure, the mutual fund. An ETF is listed on a stock exchange, so its shares trade continuously throughout the day at a live market price. You can buy or sell any moment the market is open, place a limit order, and see exactly what you paid. In practice an ETF behaves like a single stock that happens to hold a diversified portfolio inside it. Funds like VOO and QQQ have quotes that tick all day, just like Apple or Nvidia.

Most ETFs are passively managed, meaning they simply aim to mirror an index rather than have a manager pick winners. The fund publishes the rules it follows (for example, hold every company in the S&P 500 by market weight), and it buys and holds those securities. Because the strategy is mechanical, costs stay low.

What keeps an ETF's market price in line with the value of its holdings is a behind-the-scenes mechanism called creation and redemption. Large institutions known as authorized participants can exchange big blocks of the underlying stocks for new ETF shares, or hand ETF shares back for the underlying stocks. If the ETF ever trades much above or below the value of what it holds, these participants step in to profit from the gap, and that arbitrage pushes the price back toward fair value. This same in-kind process is also why ETFs tend to be tax-efficient: the fund can shed its lowest-cost shares without triggering a taxable sale, so it rarely passes capital gains on to you.

It helps to place an ETF between the two things it most resembles. Against an individual stock, the difference is diversification: a stock is one company, while an ETF is a basket of many, so a single company blowing up hurts far less inside an ETF. Both trade all day on an exchange at a live price, and both can be bought as one share, so the buying experience feels the same.

Against a mutual fund, the investment inside can be identical (both can track the exact same S&P 500 index), but the wrapper differs. An ETF trades intraday at a live price and usually has no minimum beyond one share; a mutual fund trades once a day at its net asset value after the close and often requires a $1,000 to $3,000 minimum. ETFs are generally more tax-efficient in a taxable account, while mutual funds make automatic, exact-dollar investing a bit smoother. For a deeper look, see our ETF vs mutual fund guide.

The main ongoing cost of owning an ETF is its expense ratio, the annual fee stated as a percentage of your investment. A fund with a 0.03% expense ratio costs about $3 per year for every $10,000 invested, and it is deducted automatically from the fund rather than billed to you. Broad index ETFs are among the cheapest products in all of finance, frequently charging 0.03% to 0.10%, while sector, thematic, and actively managed ETFs cost more because they are more specialized or more actively run.

Small differences compound over decades, so the expense ratio is one of the few costs entirely in your control when comparing similar funds. For the full breakdown of how the fee is calculated and what counts as high or low, see our what is an expense ratio explainer.

"ETF" is a wrapper, not a single strategy, so the label covers a huge range of funds. The most common families you will encounter:

Broad index ETFs are the workhorses that most long-term portfolios are built around. Sector and thematic ETFs let you tilt toward a specific industry or trend, but they are more concentrated and more volatile. Bond ETFs add income and stability, and international ETFs add exposure outside the US. Figures above are approximate and change; verify each fund's current expense ratio on the issuer's site. If a specific trend interests you, our ETF investing hub maps funds by category, income, account, and provider.

The appeal of ETFs comes down to a handful of durable advantages:

• Instant diversification: one share can hold hundreds or thousands of companies, cutting single-stock risk.
• Low cost: broad index ETFs often charge just 0.03% to 0.10% a year, with no sales loads.
• Liquidity and transparency: they trade all day at a live price, and most publish their holdings daily.
• Tax efficiency: the in-kind structure means ETFs rarely distribute capital gains in a taxable account.
• Low barrier to entry: you can start with one share, or a fractional share of a few dollars at many brokers.

The trade-offs are real too. An ETF still carries full market risk: a broad-market fund falls when the market falls, and diversification does not protect against a market-wide decline. Narrow sector or thematic ETFs can be as volatile as individual stocks and sometimes carry higher fees. Trading all day can tempt some investors to overtrade, and a handful of ETFs use leverage or complex strategies that behave very differently from a plain index fund, so it always pays to read what a fund actually holds before buying.

Buying an ETF works exactly like buying a stock. The steps:

• 1. Open a brokerage account at any major US broker, if you do not already have one.
• 2. Search the ticker of the ETF you want, for example VOO for the S&P 500 or QQQ for the Nasdaq-100.
• 3. Choose an amount, either a number of shares or a dollar figure if your broker supports fractional shares.
• 4. Place the order during market hours; a market order fills at the current price, a limit order lets you set a maximum.
• 5. Hold and, if you like, automate recurring purchases to dollar-cost average over time.

Most major brokers now charge zero commission on ETF trades. For a fuller walkthrough, see our how to buy an ETF and how to invest in ETFs guides, and if you are just starting out, our roundup of best ETFs for beginners.

Once you own a few ETFs, the practical questions get specific: what do my funds actually hold, do two of them overlap heavily, and how is each doing against the S&P 500? Those depend on your real positions, not a generic table. That is where Walnut fits. It connects any major US broker, then lets you ask, in plain language through Claude, ChatGPT, or a built-in assistant, what is inside your funds, where they overlap, and how each is tracking. You can build baskets around a theme, track them against target weights, and place trades that you approve. It is read-only by default. Walnut is not an investment adviser and does not tell you what to buy.