While at school, he co-created the group The Marauders (see Marauders Map) with James and their friends Remus Lupin and Peter Pettigrew. Because of Remus's condition (see werewolf), Pettigrew, Black and Potter all learned to turn into animagi; Sirius's form was a large grim-like black dog. Consequently, he was given the nickname Padfoot.
During the first war with Voldemort, Albus Dumbledore figured out that the Dark Lord was after the Potters and knew that someone in their group, the Order of the Phoenix, was a spy. Lupin and Black suspected each other, and it is fair to assume that James trusted his best friend Black over Lupin, as well.
When the Potters went into hiding, James named Black Secret Keeper and told few of this deal. At the last minute, Black, deciding that Voldemort would seek him first and still wary of Lupin, convinced Potter to name Pettigrew Secret Keeper. Pettigrew turned out to be the spy after all and gave over the Potters' whereabouts to Voldemort, leading to their death (see Harry Potter).
Black confronted him about this, enraged. Pettigrew staged a scene, cut off his finger and killed several muggles in an explosion before turning into his animagus form of a rat (see Scabbers) and escaping. Black went to Azkaban for the crimes and did not get out until he escaped right before Harry's Third Year (Prisoner of Azkaban).
Black earned the trust of Lupin, Harry Potter, and Dumbledore after explaining the story to them (they'd not been aware of the switch), but was still at large with the wizarding community. He stayed with Lupin until Dumbledore reformed the Order of the Phoenix at Voldemort's reincarnation. The group's headquarters became Black's old family home (Number Twelve, Grimmauld Place) where he was forced to stay until getting out to rescue Harry at the Department of Mysteries. He was killed by being pushed through a veil at the hands of his cousin and Death Eater Bellatrix Lestrange.
****Sirius's Role in Fanfiction****
Several fanfiction writers prefer to put Harry Potter characters together in obscure pairings. One of the most popular is Sirius Black/Remus Lupin. While many are convinced of this couple's authenticity, nothing in the books actually supports it (canon) besides the fact that the two men spent a lot of time together. In fact, Sirius is painted as a womanizer, and in Half-Blood Prince it is implied that Lupin has started a romantic relationship with a young female Auror and cousin of Black's, Nymphadora Tonks. JK Rowling has never confirmed any romantic relationship between Black/Lupin and described their hug in Prisoner of Azkaban as "brotherly." (People argue that that is not relevant evidence; Harry's point of view is not omniscient.)
That aside, I believe that the great thing about the books is how the different relationships can be interpreted, so get as slashy as you want, don't bother me none. But, yeah. He's dead. -cough-
Black stared up at him out of those sunken eyes.
"I don’t deny it," he said very quietly. "But if you knew the whole story..."
(Sirius Black in Prisoner of Azkaban)
Harry Potter's presumed dead God-father/doggy. One time best friend of James Potter and Remus Lupin, "killed" at the hands of one evil bitch of a cousin Bellatrix Lestrange. Also, hands off, as he is betrothed to the aforementioned Remus Lupin.
The Sexy Marauder
Sirius - "You can't be serious, I'm Sirius!"
James - "Sirius, seriously, that joke is really old."
Sirius - "Seriously?"
For the next two years, Sirius remained in hiding. He served as a confidant to his godson, Harry Potter, through letters and secret meetings. In Harry's fifth year, however, Sirius went to save Harry in a battle against Voldemort and some of his Death Eaters. Bellatrix Lestrange, a cousin of Sirius's as well as a Death Eater, sent a spell towards Sirius. As a result, he fell beyond a dark veil, where he is presumed to be dead.
"I trusted you," he stuttered.
The man laughed bitterly. "You were always dim-witted, Sirius."