Sirius Black, also know as Padfoot (and sometimes snuffles) is the last member of the Black family. He was in conflict with his family who believed in blood purity.
He loved is life in Hogwarts and was inseparable from his best friend James 'Prongs' Potter and friends with Remus 'Moony' Lupin and Peter 'Wormtail' Pettigrew, all Gryffindor. During his years in Hogwarts he became an Animagi, he took the form of a huge black dog. The four friends joined the Order of the Phoenix during the First Wizarding War.
He was best man at James and Lily's wedding and named as Harry's godfather. The Potters had to go into hiding from Voldemor but were betrayed by Peter. When Sirius tried to catch him Peter faked his own death framing Sirius. Sirius was arrested for the murders of Peter and twelve muggles as well as for giving the informations that lead to the murder of James and Lily. He was send to Azkaban. He kept is sanity by focusing on his innocence, but was also obssessed by Peter's betrayal.
After 12 years in Azkaban Sirius escaped to catch Peter. He wanted to kill him but Harry conviced him to keep him alive so Sirius could prove his innocence. Sadly Peter was able to escape and Sirius had to hide.
After Voldemor's return Sirius rejoined the Order of the Phoenix and offered his house as headquarters. He was killed buring a duel with his cousin Bellatrix Lestrange during the Battle of the Department of Mysteries.
more about Sirius: here