The User Interface, often abbreviated to "UI," is a term for the game controls, buttons, icons, window layout, and everything else that players manipulate to control their character or view to see the relevant stats of characters, items, monsters, and everything else in the game.
The UI for the skill tree, inventory, belt interface, and more can be seen in this screenshot from March 2009. (Everything seen here will be modified by the final game.)
See the interface article for many more details about this aspect of the game.
