Durp is commonly used as a way to greet a companion, usually when entering a room. Durp is the natural progression from other such greetings (see below). Also it can be said in approval of an opinion or statement or when showing off about something.
What is up? -> What's up? -> Wassup? -> Waddup? -> Sup? -> Durp?
1) Guy 1 *enters room*: DURP?
Guy 2: DURP!!
2) Guy 1: Lord of the Rings is epic
Guy 2: Durp.
3) Guy 1: Just completed Angry Birds and got 3 stars on every level, DURP?!