Howstuffworks.com has a fantastic evaluation of Batman's most famous arch-nemisis as well as my personal favourite as a huge Batman comic book fan. From his origins in appearance to his career of sorts throughout the comics (graphic novels such as the Joker origin story 'The Killing Joke') and in TV shows or as voiced by Mark Hamill in the animated series ('Mask Of The Phantasm' is a particular highlight).
They go on to cover his ability as a dangerously psychotic character and what Heath Ledger will bring to the role. 'The Dark Knight' is easily the most exciting film of next year to me. I don't care what everybody else says.
Read the five page feature here and be sure to check out the related stories.