"The undead are a pox upon our world, yet no one sees fit to look into their existence and find a way to rid us of them once and for all. How long until we are confronted by the terrifying specter of an undead army of skeletal warriors raised by some crazed sorcerer or would-be demigod? Do not let the scarce reports of skeleton attacks lull you into assuming we are safe; we are never safe from these unholy legions. They are coming, mark my words."— Abd al-Hazir(src)
The undead are scattered all over Sanctuary, their presence a result of Hell's influence on the mortal realm. The undead are counted among the minions of Mephisto, and eternally seek vengeance upon the living, imbued with a hatred that defies understanding. The undead loathe the living, and seek only to consume warm flesh and blood.
Zombies are counted as the lowest ranks of the undead. Above them are the fierce skeleton warriors, and even wraiths—undead in ethereal form that are manifestations of tortured souls from the planes of Hell.
Undead are particuarly vulnerable to to effects of broken bones and shattered limbs. Clubs, maces, and scepters are consequently very effective against these beings. They are also vulernable to holy magic, and the Light is anathema to these beings.
In the period following the Darkening of Tristram, the undead rose in droves to assault the living. It reached the point where people would butcher and burn the remains of loved ones and pets, lest they rise again.
Holy Bolt spell is a spell that can only harm Undead.
The Necromancer can summon undead monsters as well by using corpses.
In Diablo III, only the Witch Doctor can summon undead monsters, for example, Zombie Dogs. Some items, like Deathseer's Cowl or Azurewrath, have special properties that only work against undead, while some, like Deadeye or Corrupted Ashbringer, deal increased damage to unliving monstrosities.
Reapers use undead minions as cannon fodder of their armies, raising hordes of unliving creatures from the humans they kill. However, their undead servants are treated as Reaper type instead, being immune to the special effects that would affect any other undead.