Bodaks are the transformed bodies and spirits of those slain by nightwalkers. Their visage is so ghastly that they can kill with a look.
Liches were powerful spellcasters, usually necromancers, who unnaturally extended their lives by passing into undeath. They can only be completely destroyed if their phylactery (a magical object which contains a piece of the lich's soul) is destroyed as well as the lich itself.
Demiliches are a subset of liches who have obtained so much magical power that they no longer require a body. To become a demilich, a lich must create a number of soul gems which act as the demilich's phylactery and are often encrusted onto the single remaining body part of the lich (often the skull).
Skeltons are creatures composed solely of bone. They were used as guardians in the Elf hideout during The Hand and the Eye.
Vampires are undead who have a constant thirst for blood (though any ichor or magical life-essence will also sustain a vampire.) Unlike most undead, vampires require sleep though they are much more able to pass themselves off as living creatures.
Wights devour the life force of living creatures to assuage their hunger for the souls they have lost and can never retrieve.
Wraiths are insubstantial spirits bereft of soul and body. They remain near where their physical form fell or was buried.
Zombies are the animated corpses of living creatures which obey the commands of their creator.