They perform arithmetic or logical operations on information. They communicate information to the outside world. They move and store information in the computer's memory. The most common input device, of course, is the keyboard. But computers can accept input signals from a variety of other devices, including pointing devices such as mice and trackball.
Output devices send information to the outside world.
Most computers use a TV-like video monitor as their main output device and some kind of printers to produce paper printouts. Storage devices, including disks and tape drives, serve as long-term repositories for data.