Jackass = One who does something stupid, usually for a laugh.
Ass = One who does something stupid, usually unintentionally, and may feel somewhat guilty about it.
Asshole = One who intentionally does something mean.
Jerk = Like a jerk, but more PG.
Douchebag = One who is generally cocky and also completely self-unaware of douchebag status
Dick = Like a douchebag but self-aware. Also, a penis.