Index Funds Explained: The Simplest Way to Invest

An index fund is a fund designed to track a specific market index — for example, the FTSE 100 (the 100 largest companies listed in the UK) or the S&P 500 (500 large US companies) — by holding all, or a representative sample, of the companies within that index. Rather than trying to pick winning individual shares, you own a small slice of the whole index at once.

Why this appeals to beginners

Picking individual company shares that will outperform the market consistently is genuinely difficult — a large body of evidence suggests most professional fund managers who try to do this fail to beat a simple index over the long run, after fees. An index fund sidesteps that challenge entirely: instead of trying to pick winners, you own the whole market (or a broad slice of it), spreading your risk across many companies at once — see our later guide on diversification for why that spread matters.

Passive vs active investing

Index funds are a form of "passive" investing — the fund simply tracks its index rather than a manager actively choosing which companies to hold. "Active" funds, by contrast, employ a manager (or team) trying to beat the market through stock selection, and generally charge higher fees for that attempt. Whether active management is worth the extra cost is a genuinely long-running debate, but low-cost index funds have become popular precisely because their simplicity and low fees are easy to understand and hard to get wrong.

What "tracking" actually means in practice

An index fund aims to closely match the performance of its index, minus a small fee (the "ongoing charge" or expense ratio). It doesn't try to beat the index — matching it, minus fees, closely and reliably is the entire goal. The gap between a fund's return and its index's actual return is called "tracking difference," and lower is generally better.

Fund vs ETF versions of the same idea

Index tracking is available both as traditional index funds (bought and sold once a day at a set price) and as ETFs — Exchange Traded Funds (which trade throughout the day like a share). For most long-term beginner investors, the practical difference between the two is fairly minor; your platform's fee structure and what's actually available often matters more than the fund-vs-ETF distinction itself — we've compared the two properly in ETF vs index fund.

Choosing your first index fund

Common beginner starting points include a fund tracking a broad global index (spreading risk across many countries and companies at once) rather than a single country's market, since concentrating in one country carries more risk than a genuinely global spread. The specific index and fund provider you choose matters less than starting with something broad, low-cost, and that you understand — refining this comes later in this learning path, once you've read about diversification and asset allocation.

Fees still matter, even though they look small

Index fund fees are typically low compared with actively managed funds, but even a seemingly tiny difference in ongoing charge compounds meaningfully over decades — the same compounding maths that grows your money also magnifies every fee taken from it. Comparing the ongoing charge figure between similar index funds tracking the same index is a genuinely worthwhile five minutes before choosing one. And once you own one, the hardest part is behavioural: our guide to what to do when markets crash is worth reading before you ever need it.

This is general education, not personalised financial advice. Investing involves risk, including the risk of losing money, and past performance is not a guide to future returns. Nothing here recommends any specific investment — for anything genuinely complex or high-stakes, speak to a regulated financial adviser.