Monitors are an output device and it is used to display information in a picture form. This can be connected to your computer to be used and can also be used for other reasons such as being used as TV.
Keyboards are a typing device for computers and uses buttons/keys which input onto the computer.
Computer mouses are used as a pointing device and it detects two-dimensional motion relative to a surface. This then gets translated on to the display unit as a pointer.