The two main streams of Islam make up the Sunni and Shia. Sunni, or orthodox Muslims, represent the overwhelming majority of the global Muslim population, approximately 85% of Shia and other minorities comprising the remaining currents, ie around 15%. The countries where the population is mainly Shiite Iran are (80%), Azerbaijan (85%), Iraq (60% -65%), Bahrain (60%) and Lebanon (28%). The kharijites, which are less known, are found almost exclusively in Oman, where they form about 75% of the population.
Sunnis and Shiites differ on their vision of how to govern the ummah and the role of the imam, or religious authority within the community. These differences appear very early in the history of Islam, a result attributed to the problems generated by the legitimacy of the successors of Muhammad (as this sequence involved in a more accurate view on religion, political power ). For the Shia minority, the message of the Koran distrust esoteric look that only direct descendants of the "family of the Prophet," personified by imams, are able to interpret correctly. Therefore, within Shiism importance is given to the clergy. For the Orthodox, however, ultimately each person must interpret the divine message alone. The clergy as such does not exist and those who understand the guidelines are not considered divine message than an infallible authority.
Source : http://ca.wikipedia.org/wiki/M%C3%B3n_isl%C3%A0mic