A high-ranking cleric of the Shia Muslims is known as an Ayatollah. The word means 'sign of God'. Ayatollahs wear traditional clothes when preaching or teaching. They wear black or white turbans with the black turbans signifying they are descendants of Prophet Muhammed (SAW) through his Ahlulbayt (AS).
There is a difference between a normal Ayatollah and a grand Ayatollah. A grand Ayatollah is a marje' and gives direction to the Muslims who follow him (taqleed) in terms of religious law and fiqh. A normal Ayatollah has no right to guide people but can only choose to follow what he thinks is correct. Everybody else has to follow a marje', a grand Ayatollah.