The difference between the two groups originated with the question of succession after the death of the Prophet Muhammed and is related to differing views about political leadership for the Muslim community.
Shi'as believe that succession to political rule of the Muslim community lies only with the family and descendants of the Prophet Muhammed, starting with his son-in-law Imam Ali
Sunnis believe that the Muslim community is free to choose whoever appears to be the most qualified person and Muhammed did not appoint any particular person to be his spiritual or political successor.
Other theological differences exist between the two, many stemming from their divergent views of leadership, the role of the Imams and resulting variations in beliefs and practices.