Fighter: Fighters are, like the name implies, the fighting class. They are definitely the best in overall combat skills. They use a sword, a shield, and chainmail armor.
Wizard: Wizards are the best magic users in the game. They have limited abilities in close combat, but at a distance they can blast anything into oblivion. When cornered, they use a dagger and leather armor.
Thief: Thieves don't really have much to do in the game. There aren't any locked doors, nor is there much worth stealing. Still, their abilities might prove useful...
Paladin: Paladins are the virtuous warriors in the game. They follow the laws of honor very strictly, but besides that, they're just fighters with special abilities.
To play as a Paladin, import a character from QFG1 or 2.