An aromatase inhibitor is a compound that bodybuilders take to help control the estrogen that aromatizes from taking test and other compounds.
There are a basic 3 kinds.
Arimidex this is a lighter ai that is good for people that are on trt or just cruising at a low amount. Remember though every one reacts different to how the compounds they take aromatize so someone who takes only a small amount could get gyno and that person might need letrozole.
Aromasin or Exemestane is my personal favorite its strong but you can break down the dose and I don't have problems but everyone is different. Aromasin is said to take estro down 85% it also doesn't have a rebound effect when you stop taking it like adex does.
Aromasin is also uzed by some thru there whole pct because it is proven in studies to raise test levels.
Letrozole or Femara a very strong ai this ai is usually for those that run around a gram a week.
It is also used when you get a gyno scare its strong enough that if you take Nolvadex and letrozole you have a good chance of getting rid of the gyno that might have formed. This ai I actually keep some on hand just in case but would never use it through my cycle its extremely strong and estrogen helps build muscle so if you crash your estro your cycle is basically over.
So these are the basic 3 choose the one that fits you and make sure you do your research.
Good luck and good gains brothers