Bard: Jack of all trades, master of none. Uses music to cast spells, is often the party diplomat.
Cleric: Divine caster, usually focused on healing, buffs and destroying undead
Druid: Divine caster, focused on nature spells and shape shifting into animals. Has an animal companion.
Fighter: Fighting oriented character, usually, but not always melee. Usually wears full plate and tanks.
Monk: Melee character, focuses on mobility on the battlefield. (Think eastern warrior monks, not the western kind)
Paladin: Mixture between fighter and cleric. Can cast limited divine spells and smite evil. Limited healing ability. Is accompanied by a celestial mount.
Ranger: Dexterity based character, focused on two weapon fighting or ranged combat. Able to pick out favored enemies and have an animal companion.
Rogue: Stealth and skill based character. Deals extra damage when attacking surprised enemies. Able to detect traps and avoid damage through extraordinary reflexes.
Sorcerer/Wizard: Arcane spell casting class, able to fill a multitude of roles depending on the type of spells specialized in.