Ayatollahs?

Ayatollah is a title given to clerics who specialize in Sharia interperetation, and are therefore essentially legal scholars. A Mullah is a cleric who has a particular reputation for interperetation of the Qu'ran and Hadith, but is not a legal scholar. The word "cleric; gets used becasue westerners have a difficulty understanding a religion without a "clergy." Cleric is a catchall phrase for a Muslim who has some study of Qu'rana dn Hadith and who may preach or teach.
In addition, Ayatollah is primarily a Persian title, although it does show up once or twice in records of the Baghdad Caliphate prior to the 11th century. Grand Ayatollah is a mostly political term utilized in the late 20th and early 21st century. I consider Grand Ayatollah to have about as much valisdity as "Great Grandmaster, Supreme Grandmaster (all of them self promoted), etc.." does in the martial arts.
Have you access to The Oxford History of Islam? If not, I reccomend it.
This is a fairly good Sunni page:

Webpage Title

But take it wth a grain of salt, since it is essentially a proselytizeing page, rather than an academic source.

Thank you, Bob
I will do the research. Seems that some garner quite a lot of power.

In any vacuum, something will fill it.

In summary, there is a hierarchy in Shia clergy, based on the years/level of study by a clergy on his way to become a Scholar (Mojtahed), and later by his level of acceptance/following by peers and the community. (I am using male pronouns intentionally)

While the transition between Ayatollah and Grand Ayatollah is nebulous, it is not a self designated title, but rather gained through acceptance of peers and followers.

The following link contains pages for more in depth explanations:

http://reference.allrefer.com/country-guide-study/iran/iran73.html