What Is a Fund? Mutual Funds, ETFs, and Index Funds Explained

If you have ever wanted to own a slice of hundreds of companies at once without buying each share yourself, you have already grasped the basic appeal of a fund. A fund pools money from many investors and uses it to buy a basket of assets, so a single purchase gives you a stake in the whole basket. The idea is centuries old, but the modern versions, mutual funds, exchange-traded funds (ETFs), and index funds, can be genuinely confusing because the names describe different things. One describes a legal structure, one describes how the fund trades, and one describes the strategy. This guide untangles them so the rest of the series makes sense.

The basics

A fund is a pooled investment vehicle. Instead of you buying ten company shares and a government bond on your own, you hand your money to a fund alongside thousands of other people. A professional manager (or, increasingly, a set of rules) decides what the pooled money buys, and you own units or shares representing your proportional slice of everything the fund holds.

The three terms beginners trip over are not three competing products. They answer different questions:

• Mutual fund answers what legal wrapper is this? It is a traditional pooled fund, usually priced once a day. In the UK and Europe these are often called OEICs (open-ended investment companies) or unit trusts.
• ETF answers how is it traded? An exchange-traded fund is a fund whose shares trade on a stock exchange throughout the day, like an ordinary share.
• Index fund answers what is the strategy? An index fund simply aims to track a market index, such as the S&P 500 or the FTSE 100, rather than trying to beat it.

These overlap. An index fund can be structured as a mutual fund or as an ETF. An ETF can follow an index or be actively managed. So "index fund" and "ETF" are not opposites, and a fund can be all three labels at once: an index-tracking ETF that is also, legally, a fund. Keeping the three questions separate, wrapper, trading, strategy, is the single most useful habit when reading fund names.

The reason any of this exists is diversification and access. Spreading money across many holdings reduces the damage any single failure can do, and pooling lets a small investor buy into markets, such as government bonds or emerging economies, that would be awkward or expensive to reach alone.

Going deeper

The oldest distinction is between open-end and closed-end funds, and it still shapes how products behave today.

An open-end fund can create and cancel units on demand. When you invest, the fund issues new units and buys more assets; when you sell, it cancels units and sells assets to pay you. Because units are created or redeemed at the fund's net asset value (NAV), the price you get is tied to what the underlying holdings are actually worth. Most mutual funds and the great majority of ETFs are open-end.

A closed-end fund issues a fixed number of shares at launch, which then trade between investors on an exchange. The fund does not create new shares when demand rises, so the share price is set by supply and demand and can drift above (a premium) or below (a discount) the value of the underlying assets. UK investment trusts are the best-known closed-end structure, and persistent discounts are a recurring feature of that market.

The second big distinction is active versus passive management. An active fund employs managers who research, select, and trade holdings in an attempt to outperform a benchmark, and they charge more for the effort. A passive fund follows a rule, usually "hold everything in this index in proportion to its size," and charges very little because there is no stock-picking to pay for. Passive investing has grown enormously: global ETF assets reached roughly $21.9 trillion by spring 2026, and the bulk of that is index-tracking. Cost is a large part of the story, and we devote a later article to why.

Where ETFs changed the game is by marrying an open-end structure to exchange trading. A traditional mutual fund deals once a day at a NAV struck after the market closes; you place an order without knowing the exact price you will get. An ETF lets you buy or sell at a live price during market hours, see the spread, and use order types such as limits. That single difference, intraday tradability, drives most of the practical contrasts covered in the next article.

A note on terminology for non-US readers, because fund names travel across borders. What Americans call the "expense ratio," UK and European documents call the OCF (ongoing charges figure) or TER (total expense ratio). A US "mutual fund" is most often an OEIC or unit trust in Britain. And most exchange-traded funds available to UK, Irish, French, and Australian retail investors are UCITS funds domiciled in Ireland or Luxembourg rather than US-domiciled products, a distinction with real tax consequences that we cover later in the series.

Common mistakes

• Treating "ETF" and "index fund" as the same thing. Most ETFs track an index, but actively managed ETFs exist, and plenty of index funds are old-fashioned mutual funds. The label tells you about trading, not strategy.
• Assuming a fund means safety. Diversification reduces single-stock risk, not market risk. A fund tracking a falling market falls with it.
• Ignoring the wrapper. Two funds tracking the same index can differ in tax treatment, dealing frequency, and cost purely because one is an OEIC and one is an ETF. The wrapper is not a cosmetic detail.
• Confusing the fund with the account. A stocks and shares ISA or a SIPP is an account that can hold funds; it is not itself a fund. Beginners often blur the two.
• Chasing past performance. A fund that topped the tables last year is not more likely to do so next year, and for active funds the long-run evidence on persistence is weak.

FAQ

Is an ETF a type of mutual fund? Loosely, yes, both are pooled, open-end funds in most cases. The practical difference is that an ETF trades on an exchange throughout the day, while a traditional mutual fund deals once daily at NAV.

Do I need a lot of money to buy a fund? No. Many funds accept small regular contributions, and because ETFs trade like shares you can often buy a single share, or a fractional share where your broker supports it.

What is the difference between a unit and a share? Mutual funds and unit trusts issue "units"; ETFs and investment trusts issue "shares." The terms describe the same idea, your proportional slice of the fund.

Are index funds always cheaper? Almost always, because they avoid the research and trading costs of active management. Cost is the most reliable predictor of long-term fund returns, which is why so much money has moved to passive products.

The takeaway

A fund is simply a way to own a basket of assets through a single purchase. The jargon becomes manageable once you separate the three questions it answers: what legal wrapper (mutual fund or ETF), how it trades (once daily or on an exchange), and what it is trying to do (track an index or beat it). Almost everything else in this series builds on those three axes. Next, we put the two most common wrappers side by side and look at how ETFs and mutual funds actually differ in trading, minimums, and tax.

Educational not advice

This article is for general educational purposes only. It is not financial, investment, or tax advice, and it does not take account of your personal circumstances. Fund investing carries risk, including the possible loss of capital. Consider seeking advice from a regulated professional before making investment decisions.

Sources

• ETFGI, "Global ETF assets reach record US$21.91 trillion" (May 2026)
• PwC, "ETFs 2030: Capitalising on disruptive innovation" (2026)
• Investment Company Institute, Fact Book (fund structures and definitions)
• FCA, guidance on authorised funds (OEICs, unit trusts, UCITS)