Defined by 
FOLDOC as "The aspects of a computer system or program which can be perceived (normally seen; sometimes heard) by a human user, and the commands and mechanisms the user uses to control its operation and input data."
A GraphicalUserInterface (GUI) emphasises the use of pictures for output and a pointing device such as a mouse for input and control, whereas a CommandLine interface (CLI) requires the user to type textual commands and input at a keyboard and produces a stream of text as output.
The study of user interfaces comes under HumanComputerInteraction.
4 pages link to UserInterface:
