There are eight classes that players can adopt for boss and empire battles. At any given time, a player has an "active class", used in empire battles. When entering a boss fight, a player selects a class for that fight only.
Players gain experience points in each class separately, and can gain levels in all eight classes.
There are Achievements that can earned for levelling up in classes.
Bosses also each belong to one of the classes, which influences the amount of damage done by the player.