The marja is the most senior rank of Twelver Shia jurist, and it is unusual among religious offices in that its standing is measurable, in followers and in revenue, and has no equivalent in Sunni Islam.
Following a living jurist
Every Twelver Shia Muslim who is not himself qualified is required to follow a marja, and to follow a living one, so that the rulings of a marja lapse on his death and his followers must transfer to another. A marja issues a practical manual of rulings covering worship, transactions and much else, and he receives khums, the fifth of annual surplus income, from those who follow him, which his office spends on seminaries, students, mosques and welfare. His standing is therefore conferred by scholarly recognition and popular acceptance rather than by appointment, and several marjas normally hold the rank at once, in Najaf, Qom and elsewhere, and do not always agree.
No Sunni equivalent
There is no counterpart in Sunni Islam. A Sunni Muslim follows a school of law rather than a living person and may consult any qualified scholar, and descriptions of Sunni authority that use the language of following a marja are importing a Shia institution. The independence of the marja rests on khums, which is why the financial question is inseparable from the question of authority: a marja funded by a state would not be the same office, and the quietist tradition of Najaf and the doctrine of clerical government developed by Khomeini divide precisely over how far that independent authority extends.