A mutual fund is a type of professionally managed Investment fund that pools money from many investors to purchase securities. While there is no legal definition of the term mutual fund, it is most commonly applied only to those collective investment vehicles that are regulated and sold to the general public. They are sometimes referred to as "investment companies" or "registered investment companies". Hedge funds are not considered a type of mutual fund, primarily because they are not sold publicly.In the United States, mutual funds must be registered with the Securities and Exchange Commission, are usually overseen by a board of directors or board of trustees and managed by a registered investment adviser. Mutual funds, like other registered investment companies, are also subject to an extensive and detailed regulatory regime set forth in the Investment Company Act of 1940. Mutual funds are not taxed on their income and profits if they comply with certain requirements under the U.S. Internal Revenue Code.Mutual funds have both advantages and disadvantages compared to direct investing in individual securities. They have a long history in the United States. Today they play an important role in household finances, most notably in retirement planning.There are 3 types of U.S. mutual funds: open-end, unit investment trust, and closed-end. The most common type, the open-end fund, must be willing to buy back shares from investors every business day. Exchange-traded funds (ETFs) are open-end funds or unit investment trusts that trade on an exchange. Open-end funds are most common, but exchange-traded funds have been gaining in popularity.Mutual funds are generally classified by their principal investments. The four main categories of funds are money market funds, bond or fixed income funds, stock or equity funds and hybrid funds. Funds may also be categorized as index or actively managed.Investors in a mutual fund pay the fund’s expenses, which reduce the fund's returns and performance. There is controversy about the level of these expenses. A single mutual fund may give investors a choice of different combinations of expenses (which may include sales commissions or loads) by offering several different types of share classes.