For most investors, the simplest way to own the stock market is not to pick stocks at all, but to buy the whole basket through a fund. Exchange-traded funds (ETFs) and index funds have become the default building blocks of modern portfolios. Here is how they work and how to choose one.
What is an index fund?
An index fund holds every security in a market index, or a representative sample, in the same proportions. A fund tracking the S&P 500 owns all 500 companies weighted by size. It does not try to beat the market; it tries to be the market, minus a small fee. Because there are no analysts picking stocks, costs are very low.
The case for indexing rests on a simple observation: after fees, most actively managed funds have historically underperformed their benchmark over long periods. Low-cost index funds capture the market’s return reliably, and cost is one of the few things an investor fully controls.
What is an ETF?
An exchange-traded fund is a fund that trades on a stock exchange like a single share. You can buy or sell it any time the market is open, at a live price. Most ETFs are index funds, but ETFs also exist for sectors, countries, bonds, commodities, themes and active strategies.
ETFs vs. mutual funds
| ETF | Traditional mutual fund | |
|---|---|---|
| How you trade | Through a broker, during market hours, at a live price | Directly or via a broker, once a day at closing NAV |
| Minimum investment | The price of one share (or less with fractional shares) | Often a set minimum |
| Typical costs | Low expense ratios; small bid-ask spread | Varies widely; can include sales loads |
| Tax efficiency | Generally high in many jurisdictions due to in-kind creation | Can distribute capital gains |
| Automatic investing | Depends on broker | Usually easy |
Why fees matter so much
The expense ratio is the annual percentage of your investment a fund keeps. It sounds small, but it compounds. On $100,000 growing at 7% for 30 years, the difference between a 0.05% fee and a 1.00% fee is roughly the difference between about $750,000 and about $570,000: well over $150,000 lost to costs. Try it yourself with our compound growth calculator by subtracting each fee from the return.
Types of ETFs
- Broad market: total US market, S&P 500, total world, developed or emerging markets. The core of most portfolios.
- Bond ETFs: government, corporate, inflation-protected, short or long duration. Used for stability and income.
- Sector and industry: technology, healthcare, energy, banks. More concentrated and volatile.
- Dividend and factor: tilt toward value, quality, momentum, low volatility or high dividends.
- Thematic: AI, clean energy, robotics. Popular but often expensive and prone to buying hype at high valuations.
- Leveraged and inverse: aim for 2× or −1× the daily return. Because of daily resetting, long-term results can diverge sharply from expectations. Designed for short-term traders, not buy-and-hold.
A checklist for choosing a fund
- What does it hold? Read the index methodology and top holdings. Two “technology” ETFs can be very different.
- What does it cost? Compare expense ratios among funds tracking similar indices.
- How big and liquid is it? Larger funds with high trading volume usually have tighter spreads and lower closure risk.
- How well does it track? Check tracking difference: how far its return has strayed from the index after costs.
- Does it overlap? Owning an S&P 500 fund and a Nasdaq-100 fund doubles up on the same mega-cap names.
- How is it taxed where you live? Domicile and account type affect withholding taxes and capital gains.
How many funds do you need?
Fewer than you think. A single global equity fund plus a bond fund can form a complete, diversified portfolio. Some investors use a “three-fund” approach: domestic stocks, international stocks and bonds. Our guide to building a diversified portfolio shows sample allocations.
See the ETF market live
Our ETF heatmap groups thousands of US-listed ETFs by asset class and colours them by daily performance, a quick way to see which corners of the market are moving.
Key takeaways
- Index funds match the market at low cost; ETFs are funds that trade like stocks.
- Fees compound: small differences become large sums over decades.
- Broad, low-cost funds make an excellent core; niche and leveraged ETFs carry extra risk.
- Check holdings, cost, liquidity, tracking and overlap before you buy.
Educational content only, not investment advice. Fund names and indices mentioned are for illustration.