The input unit of the computer system is used for feeding data and instructions to the computer. These data and instructions given to the computer are called as input and the devices used for giving input are called input unit or devices.
A computer consists of an I/O system. It is used by a person to communicate with a computer. For instance, a keyboard or a mouse may be an input device for a computer, while monitors and printers are considered output devices for a computer.
In computer architecture, the combination of the CPU and main memory is considered the brain of a computer, and from that point of view any transfer of information from or to that combination is considered I/O.
Hence, the devices that are used to input data to the computer are called input unit or devices. The input devices of computer are keyboard, mouse, MICR, OCR, scanner, joystick, trackball etc.