There are two main branches of Islam - Sunni and Shi’a. Sunni is the largest branch of Islam. Shi’a is the second largest branch. Sunni and Shi’a Muslims agree on the basic principles of Islam, such as believing in one God and the importance of the prophets. They also share the same holy book - the Qur'an.
The reason there has been a split into different branches is due to differences in belief about who was the rightful successor to the Prophet Muhammad as leader of the Muslim community after his death.
Shi’a is the second largest branch of Islam and is divided into three further branches, including Twelver Shi’a - the largest group. Many Shi’a Muslims believe that Muhammad chose Ali as his successor and they do not accept the authority of the first three Sunni caliphs. In Shi'a Islam an imam is not simply a person in charge of a mosque and its congregation, but someone who has the authority to decide the meaning of the Qur’an and which laws Shi’a Muslims should follow.