The undead are unholy creatures that somehow continue to 'live' beyond their mortal demise. Undead creatures cannot be knocked out by either gas or blows to the head, and they are often highly resistant to regular weapons. All undead are affected (to varying degrees) by holy water and fire.
Revenants are the animated corpses of builders who were executed for the most grevious of heresies. Trapped in rusting chains and armour, they seek vengeance on all living things.
Ghouls were once human, but a diet of human corpses, and perhaps the tutelage of demonic spirits, mutated them into horrific bestial humanoids. Ghouls often haunt crypts, plague wards, or other places where the dead can be found.
Zombies are walking corpses that either rise spontaneously or are created by a necromancer. They are virtually mindless, but are difficult to damage with normal weapons. Regular arrows are useless against them. With enough effort they can be hacked to pieces by a sword, but the easiest way to dispatch zombies is to hit them with holy water or fire arrows.