Sirius Black is a character in the Harry Potter book series by J.K. Rowling.
WARNING! Severe spoilers from Harry Potter and the Order of the Phoenix ahead!!!
Sirius Black was born in 1958 to a pure-blood wizarding family. His parents were great followers of Voldemort and the Death Eaters.
Though his parents supported Volemort's idiology, they never became Death eaters themselves. Yet his younger brother Regulus did. He became a death eater, until in 1980 he got scared of what he was doing and tried to run away. Voldemort caught him and killed him. Sirius's mother died around 1990.
Sirius had three cousins: Bellatrix, Andromeda and Narcissa.
Bellatrix married Rodulfus Lestrange and they both became Death Eaters. Both of them were sent to Azkaban prison for the torture of Frank and Alice Longbottom. In 1996 they escaped from Azkaban and rejoined Voldemort.
Narcissa married Lucius Malfoy who became a Death Eater. They had a son Draco Malfoy.
Andromeda was Sirius's favorite cousin. She married Ted Tonks, a muggle (a person with no magical power), and they had a daughter, Nymphadora Tonks.
Sirius attained Hogwarts School for Witchcraft and Wizardry. In Hogwarts he met three good friends: James Potter, Remus Lupin and Peter Pettigrew. As the years went on the four became more and more mischievous, and together they created the marauder's map.
Remus Lupin was a werewolf. The other three decided to become Animaguses - witches or wizards who can transform into an animal.
In their fifth year they succeded: Sirius could turn into a Dog, James could transform to a Stag, and Peter Pettigrew could turn into a Rat.
The four gave each other nicknames according to what they were. Sirius was "Padfoot", James was "Prongs", Remus was "Moony", and Pettigrew was "Wormtail"
Sirius became an especially good friend of James Potter, and when in the age of 16 Sirius ran from his home because his parents were too supportive to Voldemort, he moved to live with the Potters.
In Voldemort's years of terror Sirius was an active member in the Order of the Phoenix, a group of wizards led by Albus Dumbledore who fought against Voldemort and the Death Eaters.
In 1981 Voldemort murdered James Potter and his wife Lily. When he tried to kill their son Harry he failed and the curse backfired on himself. Voldemort wasn't killed, but lost his body and lived only as a soul.
Before James's death he assigned Sirius as Harry's godfather, just in case something bad happened. After James was killed Sirius went to the house to pick up Harry, but he met there Rubeus Hagrid who told him Dumbledore sent him to bring Harry to his muggle Aunt and Uncle.
Sirius then understood Pettigrew had betrayed them and had told Volemort the Potters' hideout. Sirius found him and tried to kill him, but Peter Pettigrew escaped.
After Pettigrew ran away, the whole wizarding wold believed Sirius had been the one who told Voldemort where the Potters were hiding. Sirius was sent to wizard prison, Azkaban
The escape from Azkaban and the meeting with Harry
In the summer of 1993, Sirius found a newspaper and understood Pettigrew was still free as a Rat and at a perfect position to attack Harry Potter.
Sirius decided to escape from Azkaban. He transformed into a Dog and one night, when he was given food, he escaped.
In June 1994 Sirius made contact with Harry Potter and told him he was his godfather. He nearly captured Peter Pettigrew, but Pettigrew escaped once again, and Sirius stayed undercover.
After Voldemort's return to power
In June 1995 Voldemort retained his body with help from Peter Pettigrew. Dumbledore decided to regather the Order of the Phoenix and naturally Sirius is in it again.
In June 1996 Sirius and other Order of the Phoenix members fought Death Eaters and Voldemort in the Department of Mysteries in the Ministry of Magic. Sirius was killed by his cousin Bellatrix Lestrange.