There's been a couple of questions about what the various classes do.
Essentially, they're just comparison tools (i.e. rock<-paper<-scissors). Each class is strong against one other class, and weak against one other class.
The following is the default set up (as per RPG/formula.php):
A '+' means the header class is strong against that class (1.25 damage multiplier) and a '-' means the header class is weak against that class (0.75 damage multiplier).
Code:
Warrior
+ Thief
- Mage
Thief
+ Gunner
- Warrior
Hunter
+ Gunner
- Mage
Mage
+ Hunter
- Warrior
Gunner
+ Thief
- Hunter