See study by Z. Folejewski (1967).
Marja (Arabic/Persian: مرجع), also appearing as Marja Taqlid or Marja Dini (Arabic/Persian: مرجع تقليد / مرجع ديني), literally means "Source to Imitate/Follow" or "Religious Reference". It is the label provided to Shia authority, a Grand Ayatollah with the authority to make legal decisions within the confines of Islamic law for followers and less-credentialed clerics. After the Qur'an and the Prophets and Imams, marjas are the highest authority on religious laws in Usuli Twelver Shia Islam.
Where a difference in opinion exists between the marjas, each of them provide their own opinion and the Muqallid will follow his/her own marja's opinion on that subject.
Several senior Grand Ayatollahs constitute the hawza, a religious institution. The hawzas of Qom and Najaf are preeminent seminary centers for the training of Shia clergymen. However, there are other smaller hawzas in other cities around the world, such as Karbala in Iraq and Mashhad, and Isfahan in Iran.
However others argue that although it might seem that difference of opinion among marjas would be a source of contention, almost all marjas agree on vast majority of the rulings. There are very few rulings on which marjas differ, and even then they are quite similar to each other. For example, one marja might declare something to be wajib (obligatory), whereas another might consider it mustahab (recommended). However, it is never the case when one marja considers something wajib, whereas another considers it haram (forbidden).
Most critics agree that a marja is essential in preventing sectarianism and other differences in belief from creating conflict. Whereas differences in the beliefs of various mollanahs have historically created conflict; the marja system aids in maintaining unity in a land and preventing individuals from dividing the ummah.