"Hm, if I didn't know better, I'd swear he was trying to moon us".
He is a self-proclaimed underachiever who begins each show in school detention writing lines on the blackboard, which are known as the Chalkboard Gags. He is easily distracted by almost anything, even, strangely enough, algebraic equations. He is considerably under motivated and takes great joy in disrupting the routine at Springfield Elementary. His pranks are often elaborately complex, while his actions and speech frequently show considerable mental agility, street-smarts and understanding; so he cannot be called "stupid" per se. His best friend is Milhouse Van Houten. His antics terrify everyone, including Springfield's powerful citizen Charles Montgomery Burns.
Bart's interests include Krusty the Klown, skateboarding, reading comic books (especially Radioactive Man), terrorizing Lisa, playing video games/computer games, helping Lisa solve various problems (e.g. reuniting Krusty with his estranged father), and pulling off various pranks (such as mooning unsuspecting people and prank calling Moe Szyslak at his tavern).
It had been also suspected for a long time by viewers that Bart had Attention Deficit Disorder (ADD) even before the revealing of the Simpson gene, as Bart exhibited many of the traits of a child afflicted with ADD. His extreme mental quickness coupled with his inability to be able to keep his attention set on mediocre work at school such as in the episode "Bart Gets an F" are big hints. It was later confirmed in the episode "Brother's Little Helper" that Bart is indeed afflicted with ADD.
Whenever Homer finds out that Bart has said or done something stupid or bad, he yells out: "Why you little–!", and strangles him in anger.