Billy Bob Thornton stars as the titular bad Santa, a alcoholic, foul-mouthed, safe cracker who is hired by department stores along with Tony Cox as the brains behind the outfit. Bernie Mac, John Ritter, Lauren Graham, Lauren Tom, and Brett Kelly round out the rest of the cast, all turning in uniquely comedic performances when placed opposite Thornton.
Part of the success of Bad Santa is how dirty the childhood figure of Santa Claus becomes when occupied by an "an eating, drinking, shitting, fucking" man. Many films have had the drunk-in-Santa outfit as a passing scene2 but I can think of none that placed that character center stage. The movie also moves this Santa to Phoenix, AZ even further divorcing Santa from the snowy winter scenes in which we're used to seeing him.
I own a copy of Badder Santa, which is longer than the theatrical version of Bad Santa. According to Wikipedia,3 the directors cut is actually shorter than the theatrical version and paints Thornton as a much less sympathetic character in the end. It's difficult to imagine, but it's something I would enjoy seeing.
1. American Pie and Reno 911!: Miami had similar feelings throughout. Coincidentally I saw both American Pie and Bad Santa in theaters with my friend Corey.
2. e.g., Trading Places.