Definition
A user interface (UI) is the surface where a human interacts with a machine, covering the visible and operable elements on screen—buttons, menus, icons, text and more. It determines what a user can do, how, and what feedback they receive.
How it works
A UI organizes a machine’s capabilities into understandable controls: the user acts through input (clicking, touching, typing), the interface passes requests to the underlying program, and the results are presented back as visuals, sound or other feedback.
Usage
Every machine a person operates—from operating systems and websites to applications—depends on a UI. The same functionality can ship with different kinds of UI, such as a command line, a graphical interface or voice interaction.
Related and confusable terms
GUI is the graphical subset of UI; the broader category also includes non-graphical forms. UX is about the complete experience of using a product, of which the UI is only the part users touch directly.