Index Funds Explained for People Who Don't Know What an Index Is

If you've spent any time reading about investing, you've seen the words "index fund" thrown around like everyone already knows what they mean. Most articles skip straight to which ones to buy without ever explaining what you're actually buying.

Let's fix that. Here's what an index fund actually is, why basically every serious investor — from Warren Buffett to your financially-savvy coworker — recommends them, and how to buy your first one.

Start here: what's an index?

An index is just a list of stocks that represents a slice of the market. The S&P 500, for example, is a list of the 500 largest publicly traded companies in the United States — Apple, Microsoft, Amazon, JPMorgan, ExxonMobil, and 495 others. The index tracks how all those companies are performing together.

When you hear "the market was up 1.2% today," they're usually talking about the S&P 500.

So what's an index fund?

An index fund is an investment that simply buys every stock in an index. If you buy an S&P 500 index fund, you own tiny pieces of all 500 companies on that list — automatically, in one purchase.

That's it. It's not complicated. You're buying a little bit of everything instead of trying to pick winners.

Why this is actually genius

Here's the thing about picking individual stocks: it's extremely hard to do consistently well. Professional fund managers — people with MBAs, entire research teams, and decades of experience — fail to beat the overall market more than 80–90% of the time over long periods.

If the pros can't reliably do it, the odds that you or I can are pretty slim.

An index fund sidesteps this problem entirely. Instead of trying to beat the market, you just own the market. When the US economy grows over time — which historically it has, despite crashes and recessions — your investment grows with it.

The fee thing — and why it matters more than you think

Traditional actively managed mutual funds charge fees — called expense ratios — of 0.5% to 1.5% per year to pay the managers doing all that research. That sounds tiny until you see what it does to your returns over decades.

Say you invest $10,000 and earn 7% a year for 30 years:

Same investment, same time period — but the fee difference costs you over $17,000. That's the fee compounding against you just as hard as returns compound for you.

Index funds are cheap because there's no team of analysts to pay. The Fidelity ZERO Total Market Index Fund (FZROX) charges literally 0% in annual fees. Vanguard's S&P 500 fund charges 0.03%. That's essentially free.

What are the main types?

You don't need to memorize all of these, but here are the ones you'll encounter:

• S&P 500 index fund — tracks the 500 largest US companies. This is what most people start with and what Buffett recommends. Ticker: FXAIX (Fidelity), VFIAX (Vanguard), VOO (Vanguard ETF)
• Total US Market fund — like the S&P 500 but includes smaller companies too. Slightly more diversification. Ticker: FZROX (Fidelity), VTI (Vanguard ETF)
• Total World fund — US and international stocks combined. More diversification, slightly lower historical returns than pure US. Ticker: VT (Vanguard ETF)
• Bond index fund — bonds instead of stocks. Lower returns, lower risk. Useful as you get closer to retirement.

For most people in their 20s and 30s, an S&P 500 or Total US Market fund inside a Roth IRA is the place to start and the only thing you need for years.

ETF vs. mutual fund — what's the difference?

You'll see index funds offered as either mutual funds (like FZROX) or ETFs — exchange-traded funds (like VOO). They work almost identically for long-term investors. The main difference: ETFs trade throughout the day like stocks, mutual funds settle at end of day. For someone investing monthly and leaving it alone, it genuinely doesn't matter which you pick.

How to actually buy one right now

1. Open a Roth IRA at Fidelity (free, no minimum — see our full guide here)
2. Link your bank account and deposit any amount — even $50
3. Search for "FZROX" or "FXAIX" in the investment search bar
4. Click "Buy" and enter your dollar amount
5. Set up automatic monthly contributions so you never have to remember

That's genuinely it. You now own small pieces of hundreds of American companies, paying essentially zero in fees, inside a tax-free retirement account. Most people who've been putting this off for years wish they'd done it sooner — not because they missed a complicated strategy, but because they missed years of compound growth while waiting to feel "ready."