From WoWWiki
I am not entirely new to wikis, as I have had experience. However, my sole purpose here is to categorize Undead knowledge and lore. For me, classification goes as follows; (a friend on Warcraft 3 taught me it, and I edited it slightly)
Class 1 Undead, retaining intelligence and somewhat free will, although still bound to their master. They are able to manipulate Class 3s.
Class 2 Undead, having intelligence, although strongly bound to their master, often from religious belief. Are sometimes able to manipulate Class 3s.
Class 3 Undead, having no intelligence whatsoever. They are mindless drones, and often are controlled en masse by Class 1 and 2s
