Mojo Jojo's history is explained an episode of
The Powerpuff girls and in the
movie. It seems that he used to be a regular monkey named Jojo in
Professor Utonium's lab, though he was sorta bad, he broke the beaker of
Chemical X over the Professor's experiment, which gave
Powerpuff Girls super powers as well as Jojo's own.
The Chemical X changed him, it made his brain so big that it grew out of his skull, and made him smart, but evil. I don't want to give away the movie, but like he always does, tries to take over the world, and rule as King. In the movie you see more of his motivation and his distrust for mankind, like Magneto in X-Men.
He changes his name from Jojo to Mojo Jojo and puts a cool cap/onepiece turban on to hide his brain, wears a blue cape and white gloves.
He's a mad monkey genius, and commands with a deep voice how all should Bow to Mojo Jojo! He growls, no, bellows this at the masses as he stands on the mayor's face as a demonstration of his power.
What makes him so funny is how he repeats himself. He'll repeat himself several times, all while talking very rapidly with a vague accent. He also refers to himself constantly in the third person "Stop laughing at me! I said no laughing! Because Mojo Jojo has said so!"
He's a repeating character on the show, and usually comes up with a new scheme now and then. He has actually led to the development of the Powerpuff Girls at least twice, by adding the Chemical X, and traveling back in time to destroy the professor, which inspired him to make the "perfect little Girls" (in episode "Get Back Jojo").