Sirius Black is said to have been one of the last members born to the House of Black, a prominent family of evil sorcerers. Known members of the family include Narcissa Black, who is married to Lucius Malfoy as Narcissa Malfoy, Andromeda Black, who married Ted Tonks and had Nymphadora Tonks, Bellatrix Black, who married Rudolphus Lestrange and Regulus Black, who was killed on Voldemort's orders.
Sirius Black was best friends with James Potter, Harry Potter's father. Indeed, James and Lily Potter made Sirius Harry's godfather. Sirius is an Animagus, and went by the nickname "Padfoot" because of his form, that of a dog. When Harry Potter, Ron Weasley, and Hermione Granger communicated with him throughout Harry Potter and the Goblet of Fire (when he is in hiding), they call him "Snuffles" and pretend that he is a friendly stray dog.
Sirius' family sympathized with Lord Voldemort's desires to cleanse the wizarding world of non-purebloods, and this may have led to Sirius' incarceration at Azkaban due to accusations of murdering eleven people. (Note that Rowling has not explained why Veritaserum is not used in trials such as these; perhaps powerful witches and wizards can fool even that potion.)
During the course of events described in Harry Potter and the Prisoner of Azkaban, Sirius escapes. After a good deal of confusion, Harry comes to realise that Sirius is definitely on the side of good; however, being on the run, Sirius can communicate with Harry only sporadically.
During the events described in Harry Potter and the Order of the Phoenix, Sirius donates his family home for use as the headquarters of the Order of the Phoenix. He is effectively trapped there due to the manhunt for him, even though the Auror assigned to his case is also a member of the Order. When he leaves for a short time to see Harry to the train station on his journey to Hogwarts School of Witchcraft and Wizardry, he is recognised.
Despite this, Sirius makes a number of attempts at contacting Harry through the Floo system, which is eventually monitored by the Ministry of Magic and Dolores Umbridge. Lord Voldemort uses his growing power to influence Harry's mind, making him believe that Sirius is being held captive in the Department of Mysteries. In the ensuing battle, Sirius actually disappears; he falls through an arch and does not reappear, presumably dead. He was killed by Bellatrix Lestrange, his own cousin.