Answer:
There are two types of Muslims the Sunni and the Shites. The Prophet PBUH advised the companions that Imam Ali (as) would be the leader of muslims.
The companions were not fond of the proposal. And were telling that the Prophet is sick and he does not know what he is talking about.
They then appointed Abu Bakr as the leader and went against the Prophets words.
Imam Ali (as) objected the appointment. And was forced to give his alliagance. As he had not enough men to go to war against them, he gave alliagance for good of islam.
The Sunni's then followed Khalifa's that included people like Muawiyah and Yazid!! Who were responsible in the killings of Imam Hussein (as) the grandson of the Prophet PBUH. They also killed the son of Abu Bakr who was a follower of Imam Ali (as).
The Sunni's have different school of laws in their own sect, as the Shias do too.
The Shia truly believe that the 12 imams are the leaders of the Muslims. They included his Son in law and cousin Imam Ali (as) and the grandsons.
Shias consider the Imams Infallible, meaning they choose not to make mistakes.
The principles in Sunni and Shia are very similar, there is disagreement in leadership and a few minor things.
Most importantly they believe that Allah is the one and only and his messenger is Prophet Muhammed PBUH.