Sirius Black was one of James Potter's best friends before he was killed, and Sirius was then sent to the wizard prison Azkaban because he was the person the Ministry thought had told Voldemort where the Potter's had been hiding. But, he was innocent of this, as it was another of his and James friend, Peter Pettigrew. In the Prisoner of Azkaban, Sirius escapes Azkaban, the first person to ever do so, and goes into hiding. The Ministry then alerts everyone to be on the lookout for him, but none of them knew he was an Animagi. 

Everyone thought he was after Harry, but it turns out he was trying to save him from Pettigrew, who was also an animagi, and Pettigrew was Ron's pet Rat Scrabbers. Sirius finally finds Pettigrew, but Pettigrew manages to escape and Sirius once again has to flee for his life.

Sirius is Harry's Godfather, he gives Harry The Firebolt racing broom that Harry now use's for Quidditch, and is currently in hiding with a Hippogriff, Buckbeak.