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.

That's because Western RPGs pretty much all use D&D core rules.

What he said.