Final answer:
The Shia branch of Islam holds that the imam has spiritual and political authority in the Muslim community. Shia Muslims look to the family of the Prophet Muhammad for guidance, considering Ali as the first legitimate imam.
Step-by-step explanation:
The Shia branch of Islam believes that the imam is the spiritual and political leader of the Muslim community. The differentiation between the two main branches, Shia and Sunni Islam, occurred because of divergent beliefs about who should lead the community after the Prophet Muhammad's death. Shia Muslims believe leadership should stay within the Prophet's family and they regard Ali, the Prophet's cousin and son-in-law, as the first legitimate imam after Muhammad's death.
In contrast, Sunnis comprise approximately 83% of all Muslims and do not require that leaders come specifically from Muhammad's descendants. They choose leaders based on community consensus. The Shia sect further divides into groups such as the Twelvers, who believe in a succession of twelve imams.