Cousin and son-in-law of Muhammad. In 656 he became the last of the "rightly guided" caliphs. The fourth of the caliphs or successors of Muhammad, was born in Mecca. His father, Abu Talib, was an uncle of the Prophet, and Ali himself was adopted by Muhammad and educated under his care.
While a boy, he distinguished himself by being one of the first to declare his adherence to the cause of Muhammad. Some years afterwards he married the Prophet's daughter Fatima Zahra. Ali proved himself to be a brave and faithful soldier, and when Muhammad died without a male heir, some thought Ali to have the best claim to succeed Muhammad. Not until 656, after the murder of Uthman, the third caliph, however, did Ali assume the title of caliph. Certain conspirators later claimed that he took no steps to prevent this murder, but other sources claim that Ali sent his sons to defend Uthman, and was angered when they were unable to protect him.
The question of Ali's right to succeed to the caliphate is an article of faith which divided the Muslim world into two great sects, the Sunni and the Shia. The Sunnis believe that the prophet chose Abu Bakr to be the first caliph, while the Shia believe that he chose Ali. Ali did not challenge Abu Bakr or any of the later caliphs, however, but rather served as an advisor to them.
Ali is greatly respected by all Muslims, both Sunni and Shia. The Shia venerate him as second only to the prophet, and call him among several titles the "Leader of the Faithful" (Amir-ul-mumineen) and the "Lion of God" (Sher-i-Khuda), and celebrate the anniversary of his death. In the eyes of later Muslims he was a remarkable scholar, and he wrote many collections of proverbs and verses. The most famous collection of Ali's speeches and letters is the Nahj ul Balagha, "The peak of eloquence".
He is entombed in the grand shrine at Najaf in modern Iraq, although some believe he is buried at Mazar-e Sharif in Afghanistan. His sons Hasan and Husayn are also revered by Muslims, especially the Shia.
Sources: Saudi Aramco World, (January-February 2002); Word IQ Dictionary