Abu Bakr
Abu Bakr Siddiq (c. 573 CE – 23 August 634/13 AH), Mecca in a rich family, was the Prophet Muhammad's father-in-law, one of the closest companions and adviser. Abu Bakar succeeded to the Prophet's political and administrative functions, thereby initiating the office of the caliphate. Abu Bakr purportedly was the first male convert to Islam, but this view is doubted by a majority of Muslim historians. Upon Muhammad's death he became the first Muslim ruler (632–634), regarded in Sunni Islam as the first of the Rashidun (righteously guided Caliphs). #### In Shia Islam, he is, however, regarded as a usurper and political opportunist. His caliphate lasted two years and three months, during which time he consolidated the Muslim state.
He invaded Sassanid Persian and Byzantine Empire conquering the lands of Syria and Iraq.
Acceptance of Islam
Abu Bakr was to Sunni Islam the first “post-puberty” free male to accept Muhammad's prophethood though Shias maintain Abu Talib and other adult members of Muhammad's immediate blood family were.
Death of the Prophet
According to Sunni Muslims, Abu Bakr was the successor of the Prophet, while Shias believe that the Prophet had already appointed Ali in his lifetime as his successor.
Sunni view
According to Sunnis Abu Bakr had always been the closest friend and confidant of Muhammad throughout his life. He was always there beside the Prophet at every major event.
Shi'a view
The Shi'a have a very unfavorable view of Abu Bakr. They believe that he was a usurper who snatched the Caliphate when it should have gone to Ali, who had been appointed by Muhammad as his successor at Ghadir Khumm.
The Shi'a criticize Abu Bakr for a dispute between him and Muhammad's daughter, Fatimah, that ended with her becoming angry with Abu Bakr and refusing to talk with him for the rest of her life, she died six months later. Abu Bakr had refused to grant her a piece of land (Fadak) which Muhammad had given to her as a gift before his death. Shias also accuse him of participating in the burning of the house of Ali and Fatima, an event which is corroborated by several Sunni sources.
Sources :
wikipedia