[Spoiler alert]

I have just watched the film. It is safe to say that he is the first Joker. The first for many to come. He inspires and brings a crowd of "rich people haters" unintentionally and turns Gotham into chaos. Just my interpretation but I think Joaquin Phoenix's joker is more of a motif to be carried out through the other jokers like Heath Ledger's joker which embodies the same notion of Laughter and mental insanity.

Bruce is also shown in the movie but as a 5 year old. There is also a scene where Thomas Wayne and his wife are shot dead in the same alleyway but with a different perpetrator. This time its one of Joker's goons who are protesting against the rich. He is also hiding behind a clown mask.

To sum up, I reckin this Joker is NOT the real joker but clearly an inspiration to the upcoming Joker portrayed by Heath Ledger.