A peripheral or peripheral device is ancillary device used to put information into and get information out of the computer.

Three categories of peripheral devices exist based on their relationship with the computer:

an input device sends data or instructions to the computer, such as a mouse, keyboard, graphics tablet, image scanner, barcode reader, game controller, light pen, light gun, microphone, digital camera, webcam, dance pad, and read-only memory);
an output device provides output from the computer, such as a computer monitor, projector, printer, headphones and computer speaker); and
an input/output device performs both input and output functions, such as a computer data storage device (including a disk drive, USB flash drive, memory card and tape drive).
Many modern electronic devices, such as internet-capable digital watches, keyboards, and tablet computers, have interfaces that allow them to be used as computer peripheral devices.