The Undead

The undead are the reanimated corpses of men, elves, and orcs. They can range from mindless hordes, to being able to perform simple tasks, to retaining memories.
The mindless hordes are simply corpses animated by energy with no real willpower. Those who remember themselves, liches and vampires, are running on Lifeforce deficits and need to siphon energy from the living.
Liches in particular are constantly fighting decay and so they may attempt to transfer their soul into a living body.
Ghoul - a corpse reanimated with energy, they have no willpower. They can either be created by a necromancer, or, emerge naturally
Wight - wights can perform simple tasks and use weapons, especially useful for a labor force, or, to man crew served weapons. Orcs make the best wights and their natural healing factor allows them to function longer.
Skeleton - bones that've been reanimated with ectoplasm, they can wield weapons or be part of a horde, their only drawback is that they're very frail.
Vampire - vampires are people who suffered Witcher Rot and had their soul eroded. They often lament on their lost humanity or go stark mad. They, usually, lack any decay and can blend in.
Lich - liches are wizards who've transcended the boundries of life and used unnatural power to anchor their soul to their body. The process is incomprehensible to the layman and could drive them mad. It's entirely possible for a vampire to become a lich.
Vampires and liches often serve as monster lords who command hordes of lesser undead. All undead are affected by sunlight; ghouls and wights merely flee from it while it causes vampires to disintegrate into ash.