When using a computer the text of programs, commands to the computer and data for processing have to be entered. Also information has to be returned from the computer to the user. This interaction requires the use of input and output devices.
The most common input devices used by the computer are the keyboard and the mouse. The keyboard allows the entry of textual information while the mouse allows the selection of a point on the screen by moving a screen cursor to the point and pressing a mouse button. Using the mouse in this way allows the selection from menus on the screen etc. and is the basic method of communicating with many current computing systems. Alternative devices to the mouse are tracker balls, light pens and touch sensitive screens.
The most common output device is a monitor which is usually a Cathode Ray Tube device which can display text and graphics. If hard-copy output is required then some form of printer is used.