The keyboard is probably the easiest kind of input device to understand. When the user presses a key, a code is sent to the cpu, which translates the code into some sort of storage format (usually ASCII.) There is generally no way for the cpu to send information back to the keyboard, so it is an input only device. Usually, you buy a keyboard as part of a computer system, but there may be a reason you want a specialized model. Certain keyboards are designed to be more ergonomically safe to prevent Carpal-Tunnel Syndrome (a common malady of those who spend all day at keyboards.)