Umar ibn al-Khattab (c. 581 - November, 644), sometimes referred to as Omar, was the second caliph of Islam and one of the first four caliphs who were elected on the basis of merit.
Umar was born in Mecca around 581 to the Adi clan of the Quraish tribe. Umar belonged to a family of average class, but he was able to become literate and was well known for his physical strength, becoming a champion wrestler. When Muhammad first declared his message of Islam, Umar took it as a sacrelige upon the idolatry of the Quraish and his ancestors. Umar was well known for his hot temper, and one day he resolved to kill Muhammad. However, he was stopped on his way to Muhammad's house with news of his sister's conversion to Islam. This news detoured him to his sister's house where he found both her and her husband reading the Quran. Umar was intially angered by this and began threatening them, but their resolve and fearlessness led Umar to a change of heart. He asked to read what they were reading, and he was instantly changed. Rather than killing Muhammad, he set out to his house to accept Islam.
His conversion to the new faith also gave it some muscle due to Umar's reputation as a great warrior. Umar would be part of the first emmigration to Medina, and became an important companion of Muhammad. Umar is credited with originating the adhan, as he saw it in a dream. He also participated in all of the Muslim battles against the Quraish. Upon the death of Muhammad, Umar was in such a state of despair that he threatened to decapitate anyone who said that Muhammad was indeed dead. Abu Bakr would calm Umar and others down when he put the death of Muhammad into perspective with: If you worship Muhammad, know that he is dead; if you worship God, then know that is everlasting and will never die.
Abu Bakr would become the first sucessor to Muhammad. During Abu Bakr's short reign as caliph, Umar would be an important advisor to him, and Abu Bakr selected Umar as his sucessor prior to his death.
Umar reigned as caliph from 633 until his assasination in 644. Umar's time as caliph would see the Islamic empire grow at an unprecidented rate, taking Iraq and parts of Iran from the Sassanids (and effectively ending that empire), and taking Egypt, Palestine, Syria, North Africa and Armenia from the Byzantines. Umar also codified Islamic law, and was known for his simple lifestyle and modest living. So much so, that a famous story tells of him arriving in Jerusalem walking beside his camel upon which his servant was sitting.
Umar would die in 644, the victim of an assasin's dagger. The assasin of Umar was a Persian slave who was angered by a personal quarrel with Umar, he stabbed the caliph six times as Umar led prayers in Masjid al Nabawi. Umar would die two days later, and is buried alongside Muhammad and Abu Bakr in the aforementioned masjid. Prior to dying, he appointed a council of six men to appoint his sucessor from amongst themselves, they chose Uthman ibn Affan.