Warning: Massive spoilers for the Harry Potter series ahead.
Sirius Black is a wizard who was locked up in Azkaban, the wizards' prison, for twelve years for a crime he didn't commit: he was accused of betraying Harry's parents to Lord Voldemort and then killing his fellow wizard Peter Pettigrew and some twenty Muggles before being captured by the Ministry Aurors.
Black had been James Potter's best friend, his best man and Harry's godfather - so you can imagine the betrayal was indeed severe. Since, however, he was not guilty of any of the above, a fact of which Harry became convinced at the end of Harry Potter and the Prisoner of Azkaban, he has been watching over Harry from afar, while staying in hiding and avoiding the vindictive and still suspicious Ministry of Magic.
Sirius Black is my favourite character in the the series so far; he has a depth of character, a conflict and a darkness about him which is almost uncomfortable to find in what is after all a children's story. I like the fact that none of the gritty, unfair and monstrous details of his fate are hidden way or euphemised for the young readers, just like I like the fact that Harry and Black don't get to live happily ever after at the end of the third book. That's life, it's not nice, and I'm glad Rowling has the balls to say so.