A mutual fund is a professionally managed investment fund that pools money from many investors to purchase securities. These investors may be retail or institutional in nature. Mutual funds have advantages and disadvantages compared to direct investing in individual securities. The primary advantages of mutual funds are that they provide economies of scale, a higher level of diversification, they provide liquidity, and they are managed by professional investors. On the negative side, investors in a mutual fund must pay various fees and expenses. Primary structures of mutual funds include open-end funds, unit investment trusts, and closed-end funds. Exchange-traded funds (ETFs) are open-end funds or unit investment trusts that trade on an exchange. Some close- ended funds also resemble exchange traded funds as they are traded on stock exchanges to improve their liquidity. Mutual funds are also classified by their principal investments as money market funds, bond or fixed income funds, stock or equity funds, hybrid funds or other.