Monk
White mage
Warrior
White Mages are the healing class of the game. A white mage can heal wounds, cure poison, disease and other problems and even raise characters back from the dead. White mages also get defense spells and buffs to help other party members in a battle, and even get some offensive spells. While not essential to an experience party, the White Magebenefits the party many times over.
The monk specializes in martial arts and hand to hand combat. They wear light armor in order to retain speed and flexibility and their weapons are limited to claws and other weapons that directly attach to the hands. The Monk is a powerful offensive fighting force.
The warrior is the basic tank class of the game. Able to use almost all weapons and armor, and to taunt the enemies off of other party members, warriors are at the front of every battle. They primary role of the warrior in a group is to take the brunt of the enemy damage and keep him off the other party members
Black Mage
Red Mage
Dark Knight
The Black Mage specializes in attack magic. High in intelligence and magical ability, no other class is able to inflict magical damage quite as well as the Black Mage. However, the Black Mage is weak in combat skills and cannot take much damage in a fight. The Black Mage is most effective in a group standing back and blasting down the group's enemies.
The Red Mage is the jack of all trades of Vana'diel. He can cast both black and white magic, just not as good as the specialists in those magics. He can also wield enough weapons and wear enough armor to fight in close when necessary. He also gets his own set of abilities and spells.
The Black Knight is a combination warrior and black mage, and as such is a master at inflicting damage. He is able to wield most weapons and wear most armor and also wield basic elemental magic to inflict damage on his enemies.