It is an input device to give computer some input. It is like pen in writing. It is called keyboard because it is comprised of a board with keys installed. Every key send unique information to computer.
A keyboard on a computer is almost identical to a keyboard on a typewriter. Computer keyboards will typically have extra keys, however. Some of these keys (Control, Alt, and F Keys) are meant to be used in conjunction with other keys just like shift on a regular typewriter.
You can see some beautiful Keyboards here.