The title of ayatollah, which means sign of God, is given to Shi'a clerics who have amassed many followers and become experts in religious, ethical, philosophical, and jurisprudence matters. It is the highest rank of Shi'a theologians. Ayatollahs do not exist in the Sunni Islamic sect; Sunni Islam does not have such a formal religious hierarchy as the Shi'a but they too have religious titles such as Mufti.
In order to become an ayatollah, a Shi'a cleric must study the Qur'an and Islam for many years. The title is not bestowed in an official manner, and the Islam faith has not laid out any requirements that a cleric must meet before becoming an ayatollah. Instead, the title is earned by studying, teaching, and preaching over a long period of time and also gathering followers and garnering the respect of other Shi'a clerics.
An aspiring cleric usually starts by studying theology, science, law, and philosophy in a Shi'a holy city. After some years of studying, the cleric will begin to lecture on his or her interpretations of the Qur'an and other Islamic texts. Later, he or she will likely write books about religious topics and his or her fame will hopefully spread. He or she may become considered a marja' at-taqlid, which means "a source of emulation." After a while of this, the aspiring cleric will ideally have many followers, known as a muqallid, and be considered an ayatollah. While most are men, some women have become ayatollahs.
Once a cleric has become an ayatollah, he or she is considered qualified to issue edicts originating in the Qur'an, Sunnah, Ijmāˤ, and 'Aql, which are the Shi'a Muslims' source of religious laws. They can also teach in hawzas, which are traditional Shi'a seminaries. An ayatollah can also be called upon to act as a judge or a reference in religious matters.
An ayatollah might eventually be considered a grand ayatollah or Ayatollah Ozma, which means "great sign of God." This comes about in a manner that is somewhat more formal than the bestowing of the typical title. In this process, a council of Shi'a elders decides that an ayatollah should be considered a grand ayatollah. This process is typically initiated after he or she has become generally accepted as a reference for religious issues and has written one or more highly-referenced books about Islam. After that, he or she will be considered one of the highest-ranking experts on Islamic matters, outranked only be the Qur'an, the Prophets, and the Imams.