Unit investment trusts are investment companies that buy a specific portfolio of stocks, bonds, or other securities. They are cousins to mutual funds, but they have some fairly significant differences, too.
Mutual funds are ongoing operations that never expire.
UITs generally buy and hold a fixed portfolio of stock, bonds, or other securities, often concentrated in a particular industry or sector. This is in marked contrast to mutual funds, which are required to adhere to certain rules of diversification and must hold a minimum number of different securities. UITs are not subject to those requirements, and so can own shares of stock in just a few companies. Since a mutual fund can't own just ten different companies, a number of UITs have been created to implement the Dogs of the Dow strategy. Mutual funds can sell and buy shares frequently as long as those transactions meet the funds objectives stated in its prospectus.