A UserInterface is an umbrella term for various concepts, 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." The study of UserInterfaces is an important part of HumanComputerInteraction.
See GuiVersusCli for a comparison of the major models, the GraphicalUserInterface (GUI) and the CommandLine interface (CLI).
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