Answer:
What needs to be understood that as far as the religion is concerned they have almost everything in common (upto 90%).
However the major difference arises out of their understanding of history . They both interpret it differently. For the sunnis the history is a 'golden period' and for the Shiites the 'golden period' ended with the departure of Prophet Muhammad (Peace Be Upon Him).
The shiites believe that The Prophet's son-in-law Ali was his rightful successor whereas the sunnis believe that the Prophet's successors were the four rightful caliphs Abu Bakar , Umar , Uthman and Ali.