What is the difference between Sunni and Shia's Muslims

The primary difference in practice comes in that Sunni Muslims mainly rely on the Sunnah, a record of the teachings and sayings of the Prophet Muhammad to guide their actions while the Shiites more heavily on their ayatollahs, whom they see as a sign of God on earth

Explanation:

Muslims who believe that (Abu Bakr) should have been the Prophet's successor have come to be known as Sunni Muslims. Those who believe (Ali) should have been the Prophet's successor are now known as Shi'a Muslims.