There are three main factions in the game. West, East, and Pirate. You’ll start out on either the west or east continent. In Unchained there will be sub-factions for each continent that players on the opposite continent can join. These sub-factions have limited populations depending on which main faction is stronger or weaker. For more details, check out the 6.0.2 patch notes currently live in Korea.
Patch 6.0.2 Changes from archeage
If you obtain 3,000 infamy points (earned by PKing faction members or stealing crops in the wild owned by others) you’ll flip to the pirate faction. This mechanic may not be in 6.0, but there is a questline that changes you into a pirate (I think it starts at Freedich)
Growlgate Island is the pirate home base, and only 100 pirates can exist on each server at any given time.