The first thing to be discussed when the great prophet Mohammad (p.b.u.h.) passed away was to choose his successor in the new born Islamic society. Some accepted Abu Bakr succession, following some of great disciples' idea; others believed Imam Ali ( P.B.U.H ) is the true successor, since he is the one the prophet had chosen. In the future the former was referred to as Sunnites and the latter as Shiites.
Here the important thing to be mentioned is that the dispute between Shiites and Sunnites is not just on the matter of successor, but in each one's view, Imam serves special meaning which differentiates these two sects. To clarify the subject and the differences, let's review the meaning of Imam and Imamate.
The word "Imamate", though leads to various definitions, means leadership and Imam is the leader of a specific group in a specific path.
According to Sunnites, Imamate is a worldly way of ruling and leading the Islamic society and not a position dedicated by God. They believe since every society needs a leader and no clear way has been introduced to choose one in Islam, the prophet's successor can be chosen through different ways, as referring to people's or the dignitaries' votes or the last will of the previous caliph or even coup d'etat.
But Shiites consider Imamate as the continuation of prophet hood and Imam as the sign of God among people and the connection between all creatures and their creator. They believe Imam should be chosen by God Himself and be introduced by the prophet. This idea originates in the high place Imamate takes in Shiism, for Shiites consider Imam as the leader and manager of the Islamic society, the interpreter of holy Koran, the one who expresses divine rules and who leads to the way of salvation as well. In other words, Shiites regard Imam as the reference both in worldly and religious affairs, where the Sunnites believe him as to be just a caliph who takes care of the worldly affairs of people.