Jason Todd was a Robin but no one liked him and so people phoned in to have him killed. He went to find his mother in the middle east, ran into the Joker, tried to solo him, and Joker beat him down with a crowbar and blew him up.
I used to think that the Joker was some pussy foot villain who was just insane but the guy is simply the best villain of all time (with Luthor imo). Beating Robin with a crowbar, paralyzing Gordan\'s daughter, he just does it to toy with Batman and for the fun of it. His master plot usually comes back to haunting Batman, not some silly plan take over the city plan. Bat fights for justice and to make Gotham a better place to live, Joker is his opposite in every conceivable way.
And yeah I hope they have a good explanation for bringing Todd back. Like a clone or something to haunt Batman because that mother fucker should of stayed dead.