There was only one madh-hab (school of fiqh) during the time of the Righly-guided Caliphs. With the emergence of the Umayyad rule, the situation changed. The Umayyad caliphs did not have the same religious authority as the previous ones. After the Umayyad (661-750 CE) came the Abbasids. In comparison to the Umayyads, they were more supportive of Islamic law. The crystallization of four major Sunni madhahib of Islamic fiqh came about by the third century of Hijrah; before this there were about twenty different madhahib.
In the Sunni world there are now Four Orthodox Schools (Schools of Fiqh) of thought [the Four Madhahib]: the Shafi'i, Hanafi, Maliki and Hanbali. With regard to legal matters, these four orthodox schools give different weight in legal opinions to prescriptions in the Quran, the hadith or sayings of the Prophet Muhammad, the consensus of legal scholars, analogy (to similar situations at the time of the Prophet), and reason or opinion. Towards the end of the first century of Islam, Imam Abu Hanifa in Kufa and Imam Malik in Madina founded mazahib (schools) or religio-legal thought, named after them as the Hanafi and the Maliki schools. In the following century, the two other great schools were founded -- the Shafei school of Imam Idris al-Shafei in Egypt and the Hanbali school of Imam Ahmad ibn Hanbal in Baghdad. The differences between the four famous Jurists Imaam Abu Hanifa, Shaaf'ee, Maaliki and Hanbaliy stem from their differences on principles. The basic principle according to Imaam Maaliki is to prefer Amal-e-Madinah, that is the practices of the people of Madina Munawwarah. However, that principle is not adopted by Imaam Ahmad ibn Hanbal.
The fanatical loyalty to a particular madh-hab among Muslims is decreasing. Now Hanafi, Shafi`i, Maliki and Hanbali and even Ja`fari followers pray together and work together. Most scholars hold that it is not required of the Muslim to follow a certain Fiqh School because nothing can be made required of Muslims except that made by Allah and His Prophet. When in need of Fatwa, Muslims could consult with any scholar regardless of his Madh-hab. A common Muslim is said to have no Madh-hab.
Sunni Islam does not possess clerical hierarchies and centralized institutions. The absence of a hierarchy has been a source of strength that has permitted the faith to adapt to local conditions. However, it also has been a weakness that makes it difficult for Sunni Muslims to achieve any significant degree of solidarity. Despite some very minor disputes there are many Sub-Groups in the four groups like Kharjiites, Wahabis, Deobandi, Barelvi, Ahle-Sunnat Wal Jamat, Ahle Hadith, Ghurba Ahle Hadits, Sunnis of Green Turban, Sunnis of Brown Turbans etc. etc. They declare each other wrong and seldom offer prayer behind each other.
Among Sunni Muslims, effective power and the ability to maintain order are sufficient for legitimate authority, in stark contrast to the more uncompromising Shia views of government as the sole province of religious leaders. For Sunnis, even a bad Muslim ruler is preferable to chaos and anarchy, and the Sunni religious tradition contains only a limited right to rebel. However, if a ruler commands something that is contrary to God's law, the subject's duty of obedience lapses.
Originally political, the differences between Sunni and Shia interpretations rapidly took on theological and metaphysical overtones. In principle a Sunni approaches God directly; there is no clerical hierarchy. Some duly appointed religious figures, however, exert considerable social and political power. Imams usually are men of importance in their communities but they need not have any formal training; among the beduins, for example, any tribal member may lead communal prayers. Committees of socially prominent worshipers usually run the major mosque-owned land and gifts. In many Arab countries, the administration of waqfs (religious endowments) has come under the influence of the state. Qadis (judges) and imams are appointed by the government.