||The introduction to this article does not have enough information for people unfamiliar with the subject. (November 2017)|
Mutual funds are usually "open ended", meaning that new investors can join into the fund at any time. When this happens, new units, which are like [[Share (finance)[shares], are given to the new investors.
There are thousands of different kinds of mutual funds, specializing in investing in different countries, different types of businesses, and different investment styles. There are even some funds that only invest in other funds.