From Diablo Wiki
Undead, along with Animals and Demons are the three classifications of monsters found in Diablo II. Undead take 50% extra damage from blunt weapons, and can be resurrected by Greater Mummies. Monsters classified as Undead can be immune to Magic damage. However they are never immune to the paladin magic skills Holy Bolt, Blessed Hammer and the Holy Bolts that are emitted from a Fist of the Heavens attack.
See the Category:Undead for a full listing of undead monsters found in Diablo II.