The portion of a program with which a user interacts. Types of user interfaces, or UIs, include command-line interfaces, menu-driven interfaces, and graphical user interfaces. Acronym: UI.
In computing, the procedures and methods through which the user operates a program. These might include menus, input forms, error messages, and keyboard procedures. A graphical user interface (GUI or WIMP) is one that makes use of icons (small pictures) and allows the user to make menu selections with a mouse.
The study of the ways in which people interact with computers is a subbranch of ergonomics. It aims to make it easier for people to use computers effectively and comfortably, and has become a focus of research for many national and international programs.