The creator of Islam Religion who is a unique Prophet called Mohammed. He was dead in A.C. 632. Since he thought that he is the only one Prophet of the Islam, he did not settle a successor to manage the Islam. The remaining believers set up a position called Caliph to lead Islam Religion.
Mohammed
The main problem was who can be the Caliph. Some of the believers thought the believers who have blood relationship with Mohammed can be Caliph. Some of them thought the most powerful and talented believer should be the Caliph no matter the blood relationship. As a result, the former was defined as Shiite， the latter was defined as Sunni.
(If I get anything wrong, please give me comments )