What is a mutual fund? Plain English

You've heard the words “mutual fund” on every retirement form you've ever signed. But what actually is one? Here it is in the plainest words possible.

A mutual fund is a big shared pot of money. Lots of people put money in, and a manager uses the whole pot to buy a bunch of investments at once. You own a slice of the pot.

The one-sentence version

A mutual fund pools money from many people to buy many investments together, so each person gets a little piece of all of them instead of having to buy each one alone.

Why people use them

Buying 500 different stocks yourself would be a nightmare. A mutual fund does it for you in one purchase. You instantly own a spread of investments, which is far steadier than betting on one. (See: diversification.)

The catch: fees

Someone runs the fund, and they charge for it. That fee is small per year but it eats into your returns every single year, so it adds up. Low-fee funds keep more of the growth in your pocket. (See: expense ratios.)

Mutual fund vs index fund

An index fund is a type of mutual fund (or ETF) that just copies a whole market instead of paying someone to pick winners. It usually charges far less — and over decades, that matters a lot. (See: index funds: boring beats clever.)

The honest takeaway

A mutual fund is a shared pot that buys many investments at once, so you don't have to. Just watch the fees — a cheap, plain fund usually beats a pricey, clever one over the long run. Over long stretches of history, a broad basket of U.S. stocks has grown about 10% a year on average. That's the past, not a promise. (See: what “the S&P 500 averages 10% a year” means.)