Skeletons are Undead. As iconic as Zombies, animated human skeletons have been featured in all of the Diablo games.
In Diablo III, plain skeleton warriors are joined Skeletal Summoners, Skeletal Archers, and Skeletal Shieldman, and (perhaps) other types not yet revealed. These skeletons spawn in mixed packs and work together with much better AI than they have shown in the previous games in the series.
The official bestiary has an entry on skeletons, from which the following excerpt is taken:
Despite the fact that skeletons seem mischievous or imbecilic, they are nothing to scoff at. We should neither dismiss them from our consideration, nor ignore the problem they represent. It is far past the time for serious inquiry into their exact nature. Since I am overly qualified on many subjects pertinent to this area of inquiry, apparently it falls to me to rectify this lack of understanding. After completing many months of long, arduous study, I now present the information I have gleaned from my research into these unholy monstrosities.Contrary to what I had assumed, a reanimated skeleton is actually constructed from bits and pieces of any number of different skeletons, not a single one. Their diverse composition gives them the ability to form and reform, and makes them easily summoned, permitted there is adequate raw material at hand. Still, this is not to say that a skilled necromancer could not call forth a cadre of skeletal warriors to do his bidding anywhere he chooses. He merely requires less effort to construct a skeleton army in a graveyard than in the middle of a forest. Furthermore, I have come to believe that a skeleton's intelligence is limited by the power and scope of the spell used in the creature's creation. Theoretically one could have a single astute skeleton servant or a rather dense army of a hundred for the same expenditure of magical energy. I am at a loss to explain the average skeleton's somewhat ludicrous mental predisposition, however. Perhaps the implausibility of its own existence makes the skeleton think it hilarious to hide in a barrel, cackling intermittently for some three hundred years until a victim happens by?