What actually is an index fund?
And why do we keep banging on about them.

When we first started looking into investing, "index fund" came up in every article, every YouTube video, every Reddit thread. Nobody ever explained what one actually was. They just assumed you knew.

We didn't. This is the explanation we wish we'd had.

Start with the index part

An index is just a list. The S&P 500 is a list of the 500 largest companies listed on US stock exchanges, Apple, Microsoft, Amazon, Nvidia, and 496 others. The FTSE 100 is a list of the 100 largest companies on the London Stock Exchange. The index tracks how those companies are collectively performing. When people say "the market went up 1% today," they usually mean an index like this moved 1%.

Indexes aren't things you can buy. They're measurement tools. A fund is what you buy.

Now the fund part

A fund pools money from thousands of investors and uses it to buy assets. An index fund specifically buys the assets that mirror a particular index. A Vanguard S&P 500 index fund buys shares in all 500 companies on the S&P 500, in the same proportions that they appear in the index. When the index goes up, your fund goes up. When it drops, your fund drops.

You're not picking winners. You're buying the whole list.

Why does that matter?

Because picking winners is hard. Fund managers who try to beat the market, choosing individual stocks they think will outperform, fail more often than they succeed. Study after study shows that over 10+ years, the majority of actively managed funds underperform a simple index fund that just tracks the market.

Not because those managers are bad at their jobs. Because markets are efficient. The price of a stock already reflects what millions of informed people think it's worth. Consistently knowing better than all of them, year after year, is genuinely difficult.

Index funds don't try to beat the market. They just own it. And owning it turns out to work pretty well.

What's an ETF then?

ETF stands for Exchange Traded Fund. Most index funds you'll come across as a UK beginner are structured as ETFs, which just means they're traded on a stock exchange like an individual share, you can buy and sell them throughout the day. The underlying thing (a bundle of stocks mirroring an index) is the same. ETF is the wrapper, index fund is the strategy.

When we talk about our Vanguard S&P 500 investment, we're talking about an ETF that tracks the S&P 500 index. Both terms are accurate.

Why we use one

We tried picking individual stocks first. We bought Virgin Galactic at what we thought was a good price. We bought T2 Biosystems on a tip. We spent time reading about companies, watching charts, second-guessing ourselves constantly. It was stressful and we weren't good at it.

Switching to a single S&P 500 index fund was boring by comparison. The stress went away. The checking-the-app-twenty-times-a-day went away. The portfolio kept growing.

Index funds aren't the only way to invest. They're a sensible starting point for most people who want to build wealth without making it a second job.

The fees are low, the diversification is instant, the track record across decades is strong. For a beginner, that combination is hard to beat.

A few things to know before you buy

• You still need the right account. Buying an index fund inside a Stocks and Shares ISA means your gains are tax-free. Buying it outside one means they aren't. The fund itself doesn't change, the wrapper around it does.
• There are hundreds of index funds. S&P 500, FTSE All-World, FTSE Global All Cap, MSCI World, each tracks a different index. The differences matter. We cover this in The Starting Line.
• It's not a savings account. Your money goes up and down. You need to be comfortable leaving it alone for several years for the ups to meaningfully outweigh the downs.
• The fee is ongoing. A 0.07% OCF means you pay 70p a year for every £1,000 invested. Low, but worth knowing.