The Shi'a branch of Islam has its roots in disagreements about the succession to the Prophet Muhammad. One group of companions held that Ali ibn Abi Talib (the Prophet's cousin, son-in-law, and father of his only bloodline), had been designated by Muhammad as his successor, and that community leadership should be inherited through the family line - the Imams.
Zaydis believe there were five Imams, four shared with other Shi'a. They are primarily based in Yemen.
Ismailis believe in seven Imams, sharing a common lineage of six with most Shi'a. The followers of the Aga Khan are the largest group among Ismailis.
The most prominent, and largest, Shi'a group (the Ithna'sheri or Twelvers) believe there to have been a lineage of twelve Imams. They form the majority of the populations in Iran and Iraq, and substantial minorities in several other Muslim countries.
Shi'a Islam has developed a distinctive theology, jurisprudence, and spirituality. Between 10 and 15% of the world population of Muslims are Shi'a.