mutual fund is a professionally managed investment fund that pools money from many investors to purchase securities Mutual funds are often classified by their principal investments: money market funds, bond or fixed income funds, stock or equity funds, or hybrid funds Funds may also be categorized as index funds, which are passively managed funds that track the performance of an index, such as a stock market index or bond market index, or actively managed funds, which seek to outperform stock market indices but generally charge higher fees. Primary structures of mutual funds are open-end funds, closed-end funds, unit investment trusts.