The separation of the two groups started after the death of the Prophet Muhammad. The majority believed that his rightful successor was his father-in-law and close friend, Abu Bakr, but a small group believed the Prophet Muhammad’s successor should be Ali ibn Abi Talib, his cousin and son-in-law and father of his grandchildren.
The Sunni majority got their way, as Abu Bakr became the first Muslim caliph and successor of the prophet. Although the divide was at first mostly political, as the minority group was a faction supporting the power of Ali, over time, the divide evolved into a religious movement.
One of the most crucial differences between Shia and Sunni Muslims is the importance that the Shiites give to Ali, whom the Sunni do not recognize as being the prophet’s rightful successor.
One of the most important dates for Shia Muslims is the tenth day of the holy month of Muharram (the first month in the Islamic lunar calendar). Shia Muslims celebrate the anniversary of the death of Husayn ibn Ali, the grandson of the Prophet Muhammad and son of Ali.
Both groups, however, agree that Muhammad is God’s messenger and follow the five ritualistic pillars of Islam, which include fasting during Ramadan, five daily prayers, and the Hajj, an annual pilgrimage to Mecca
They also both share the holy book of the Quran. 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.
The conflicts in Iraq and Syria are also show a divide between the two, as many Sunni men have joined rebel groups, while men from the Shia community are most often fighting for or with government forces,
Between 10% and 13% are Shia Muslims, and 87% to 90% are Sunni Muslims. Sunni Muslims are also present in more countries and regions throughout the world, whereas most of Shia Muslims live in four countries: Iran, Pakistan, India, and Iraq