A collective investment scheme is a way of investing money with others to participate in a wider range of investments than feasible for most individual investors, and to share the costs and benefits of doing so.

Terminology varies with country but collective investment schemes are often referred to as mutual funds, investment funds, managed funds, or simply funds (note: mutual fund has a specific meaning in the US). Around the world large markets have developed around collective investment and these account for a substantial portion of all trading on major stock exchanges.

A mutual fund is a professionally managed type of collective investment scheme that pools money from many investors and invests typically in investment securities (stocks, bonds, short-term money market instruments, other mutual funds, other securities, and/or commodities such as precious metals).
The mutual fund will have a fund manager that trades (buys and sells) the fund’s investments in accordance with the fund’s investment objective.
In the U.S., a fund registered with the Securities and Exchange Commission (SEC) under both SEC and Internal Revenue Service (IRS) rules must distribute nearly all of its net income and net realized gains from the sale of securities (if any) to its investors at least annually. Most funds are overseen by a board of directors or trustees (if the U.S. fund is organized as a trust as they commonly are) which is charged with ensuring the fund is managed appropriately by its investment adviser and other service organizations and vendors, all in the best interests of the fund’s investors.

From wikipedia & investopedia.