Okay, so there are 4 main factions in the game, but 3 are in play at any time.
The Blue Dragon
The Unseen
The Cult
Neutrals
The Blue Dragon are the town of this game. They work as any other town, and win if they eliminate the Unseen, the Cult and any hostile Neutrals.
Either the Unseen and the Cult are in play at any one time. They are both factions with the ability to both kill and convert, but each have different mechanics.
The Unseen have two main classes: The Mastermind and The Assassin. The Mastermind may convert every other night, and the Assassin is responsible for killing. They have other abilities, but I’m afraid you have homework to do there. Every BD class that is convertable has a unique Unseen class that they will become after being converted.
I don’t know how the Cult work in FoL, someone else better talk about that, but they do work differently.
Neutrals are neutrals, they have different mechanics depending on their class, and different wincons.