A trackball is an input device used to enter motion data into computers or other electronic devices. It serves the same purpose as a mouse, but is designed with a moveable ball on the top, which can be rolled in any direction. Instead of moving the whole device, you simply roll the moveable ball on top of the trackball unit with your hand to generate motion input.
Trackballs designed for computers generally serve as mouse replacements and are primarily used to move the cursor on the screen. Like mice, computer trackball devices also include buttons, which can serve as left-click and right-click buttons, and may also be used to enter other commands. While trackballs are most commonly used with computers, they may also be found in other electronics, such as arcade games, mixing boards, and self-serve kiosks. These devices often have trackballs that are larger than the ones used in computer input devices.
Besides the capability to be built into various devices, trackballs have a number of other advantages over mice. Some advantages include the small footprint (since they don't require a mousepad or large area to move the mouse), fingertip control (which may offer more accuracy), and improved ergonomics (since there is less strain on the wrist). Still, many people find trackballs harder to use than mice, since they feel less natural and may require practice to get used to. For this reason, the vast majority of computers include a mouse, rather than a trackball, as the default input device.