First of all, the story: a fairly generic Batman plot where a bad guy falls into a pit of acid and turns into the Joker, a homicidal psycho with a taste for theatricality. The rest of the movie is slightly boring, and disturbing sometimes, but also sometimes enjoyable with some good acting or a spot of humor, although the action scenes aren't quite up to what you'd expect from a superhero movie.
Violence: there's no lack of it. Multiple people are shot, although we rarely see blood. The most graphic instance is when the Joker fires about 15 bullets into a man. We only see the first few hit him, but we later see blood on the newspaper that had been in front of him. The Joker stabs a man in the neck with a quill pen (bloodless), electrocutes a man to the point where he is a charred, blackened, bloody corpse, shoots acid at people, and uses gas that causes people to laugh to death, literally. Before he becomes the Joker, he is shot in the face (fairly bloody). There's a lot of punching, sometimes with blood, and a couple of people take long, fatal falls.
Sex: it is implied that Bruce and Vicky spend the night, along with some double entendres.
Language: there is some fairly course swearing (worst instances being s**t and g**d**n). There is some drinking.