What is UI?

Short for user interface, this is where the user may interact with a computer or computer program.

The interface is a set of commands or menus which is typically command-driven, this is basically where the user is entering instructions.

Everything from a monitor, keyboard, to a mouse or light pen can be considered to be a UI, but typically it is the term now used when referring to the graphical user interface, or GUI.

Why are UIs constantly redesigned?