The following diagram shows each class, and the classes it can progress directly into. It is possible, though rare, to cross from one path into another.
Adventurer: The Adventurer is the base class from which each of the four class paths deviate.
Knight: The Knight path, with strong melee attacks and high health, is for front line fighters. Most classes in this path have shields which can block projectiles. The knight should be at the front of the party to absorb attacks and push back monsters.
Acolyte: The Priest path is for healers, and is very important. In a full team of five, there should at least be one full time healer, and one or two backup healers. Dedicated priest also have the ability to turn undead and cast out demons, while rogue priests focus on buffing.
Mage: The Mage path is for blasting monsters from a distance, and has access to the most powerful attacks in the game. As ranged fighters with low health, mages should stay away from enemies and close to the healer. Rogue mages focus on summoning and transforming.
Rogue: Rogues don't so much have a path of their own, as they are a way to augment the other paths. Rogue classes tend to be faster, have long ranged attacks, and fight in non-standard ways. Rogue Knights give up their shields in return for more brutal attacks. Rogue Priest give up healing in return for useful buffs. Rogue Mages can summon or transform into powerful monsters. Pure Rogues are highly effective at quickly clearing the stage of weak enemies, but have low health and are very vulnerable. All highly effective teams make use of Rogues, but a team too heavy on Rogues, without enough healers, will have a very short future.