My understanding is it's a "decent into madness" kind of movie. Batman pretty much just said "The Joker is crazy", with just a little backstory.
Someone apparently asked "What could make a man so very crazy?" and made a movie. It's not really my cup of tea in any fashion (Not really A DC fan, not a fan of the Joker as a Batman villain, and not really into studies of madness type movies) so I won't be watching it.
That said the snipits I've seen on trailers seem well done, and Joaquin Phoenix is a talented actor, so it could very well be a good film of it's type. I suspect that Batman fans will be disappointed and fans of Monster will like it.