The User Interface (UI) is the view of a product or service presented to the user.

The UI of a door is usually a big surface with a doorknob, and optionally a window, a mailbox slot, a peephole viewer, etc.

Today's computer interfaces are a monitor screen, keyboard, and mouse. Monitors have been capable of displaying high-resoultion graphics (even television-quality video) for some time, so the interface has been dubbed a GUI (GraphicalUserInterface?, pronounced "goo-ey").

The design of the UI is the work of User Interface Engineers, including Visual Design? and User Experience engineering.

The User Interface of a Web Content Management System is usually a complex of web pages.

References:
Jared Spool's UI Conferences