What Is an Index Fund? A Plain-English Explanation
Understand what index funds are, how they work, why they're popular with beginner investors, and how they compare to other investment options — all in plain English.
You’ve probably heard the advice: “just invest in index funds.” Maybe it came from a friend, a personal finance article, or someone on the internet who seemed very confident about it. But if you’re not sure what an index fund actually is — or why everyone keeps recommending them — you’re not alone.
This guide breaks it down in plain English, no finance degree required.
What Is an Index Fund?
An index fund is a type of investment fund that tracks a market index. To understand that, let’s start with what a market index is.
A market index is simply a list of companies grouped together by a specific rule. The S&P 500, for example, is an index made up of the 500 largest publicly traded companies in the United States — companies like Apple, Microsoft, Amazon, and hundreds of others. The Total Stock Market index takes it even further, covering essentially every publicly traded US company, large and small.
These indexes are designed to represent a broad slice of the market. They go up when companies in the index perform well, and down when they don’t.
An index fund’s job is to mirror one of these indexes as closely as possible. When you invest in an S&P 500 index fund, you’re not just buying stock in one company — you’re buying a tiny piece of all 500 companies in that index at once.
Think of it this way: instead of betting on a single restaurant succeeding, you’re buying one slice of an enormous buffet that includes every restaurant in town. Some will have a bad year, others will thrive, but you’re not dependent on any single one to make or break your investment.
How Do Index Funds Work?
Here’s where index funds differ from other types of investment funds: there’s no team of analysts sitting in a room deciding which stocks to buy. The fund simply buys all — or a representative sample — of the stocks in the index it’s tracking. When a stock is added to the index, the fund buys it. When one is removed, the fund sells it. It’s automatic.
This is what makes index funds a form of passive investing. Instead of a fund manager actively researching companies, making predictions, and trying to pick winners (active investing), the index fund just follows the rules of the index. No guesswork, no strategy, no expensive research team.
Your returns as an investor will closely match the performance of the index itself, minus a small fee for running the fund. If the S&P 500 goes up 10% in a year, an S&P 500 index fund should return something very close to 10%.
And over the long term, compound interest makes these returns grow significantly — your gains earn gains, which earn more gains, building on themselves year after year.
One important note: while markets have historically trended upward over long periods, past performance doesn’t guarantee future results. Investing always carries risk, and your balance will go up and down in the short term.
Why Are Index Funds So Popular?
Several factors have made index funds the go-to recommendation for everyday investors.
Low fees. Every investment fund charges a fee to cover operating costs, called an expense ratio. It’s expressed as a percentage of your investment per year. Actively managed funds often charge 0.5% to 1% or more annually. Many index funds charge less than 0.1% — some even less than 0.05%. That might sound trivial, but over decades, even a small difference in fees can cost you tens of thousands of dollars in lost growth.
Built-in diversification. Diversification means spreading your money across many different investments so that a bad outcome for one doesn’t sink your entire portfolio. When you invest in a single company’s stock, your fortunes are tied to that one company. When you invest in an index fund tracking hundreds of companies, a single company struggling has very little impact on your overall return. Index funds give you instant diversification without any extra effort.
Strong long-term performance. This might be the most compelling argument for index funds: over long periods — 10, 20, 30 years — the majority of actively managed funds tend to underperform their benchmark index after accounting for fees. Fund managers may beat the market in a given year, but doing so consistently over decades is extremely rare. Index funds don’t try to beat the market; they aim to match it, and that turns out to be a winning strategy more often than not.
Simplicity. There’s no need to research individual companies, read earnings reports, or follow financial news. You pick a broad index fund, contribute regularly, and let the market do its thing over time. For most people, this low-maintenance approach fits their life far better than actively managing a portfolio of individual stocks.
Warren Buffett — widely regarded as one of the greatest investors of all time — has repeatedly said that for most people, a low-cost S&P 500 index fund is likely to produce better results over time than trying to pick individual stocks or paying for active management.
Index Funds vs Other Options
It helps to understand how index funds stack up against the alternatives you’ll encounter.
Index funds vs individual stocks. Buying individual stocks means you’re concentrating your money in a small number of companies. The upside: if you pick the right one, the gains can be substantial. The downside: if you pick the wrong one, you can lose a lot. Index funds spread that risk across many companies, which generally means less volatility — though also less potential for outsized gains on a single bet.
Index funds vs actively managed mutual funds. Both are pooled investments where many investors contribute money that’s invested collectively. The key difference is cost and strategy. Actively managed funds employ professional managers who try to beat the market; index funds don’t try, and charge far less for it. As noted above, most actively managed funds don’t consistently beat their benchmark over the long run, making the higher fees hard to justify for many investors.
Index funds vs ETFs. This one trips people up because the two are very similar. ETFs (exchange-traded funds) and index funds often track the same indexes and have similar costs. The main practical difference is how they’re traded. ETFs trade on an exchange throughout the day like a stock; traditional index funds are priced once per day. For long-term investors who contribute regularly and don’t trade frequently, this distinction rarely matters in practice. See index funds vs ETFs for a more detailed comparison.
Index funds work especially well when combined with a consistent contribution strategy. Dollar-cost averaging — investing a fixed amount on a regular schedule regardless of market conditions — pairs naturally with index fund investing and takes the guesswork out of timing the market.
How to Get Started With Index Funds
Getting started is more straightforward than many people expect.
Open a brokerage or retirement account. If you have access to a 401(k) through your employer, check whether it offers index fund options — many do. You can also open an IRA (Individual Retirement Account) or a regular taxable brokerage account on your own through any major brokerage.
Look for funds that track broad indexes. Funds tracking the S&P 500 or the Total Stock Market are common starting points. These give you wide exposure to the US economy with a single fund.
Check the expense ratio. This is the annual fee, expressed as a percentage of your investment. Lower is better. Look for funds with expense ratios well below 0.5%. Many broad index funds charge 0.03% to 0.10%.
Set up automatic contributions. One of the best things you can do is automate your investing. Decide on an amount you can contribute each month and set it up to happen automatically. This removes the temptation to time the market and keeps you building toward your goals consistently.
For a broader introduction to getting started as an investor, see our full investing for beginners guide.
Before you start putting money into index funds, it’s worth making sure your financial foundation is solid — a clear budget, an emergency fund, and a good sense of your monthly cash flow. wealthmode helps you track your finances so you know exactly how much you can confidently put toward index funds each month, without guessing or overcommitting.
This article is for informational purposes only and does not constitute financial advice. Investing involves risk, including the possible loss of principal. Consider consulting a qualified financial professional before making investment decisions.