Mice and trackballs are members of a class generically described as pointing devices. All pointing devices have the same purposeallowing you to move the cursor (or pointer) around the screen and to click to select items or perform other functions. A great variety of fiendishly clever pointing devices are built into notebook computers, but nearly all desktop systems use mice. Those few that don't, use a trackball, which are essentially belly-up mice with their balls showing.