There are three basic types of brakes use in fly reels.
1. A pawl and gear drag.
2. A caliber drag.
3. A disc drag.
There are some variations on these drags and there is also a fluid drag that is used on some Cortland reels. But these three types of drags are on the majority of reels. Here is an Article
by Zack Matthews that tells you all about drags. Read the article and then let us know if you have any specific questions.