Undead have no metabolism, are immune to the Alteration and Command effect groups, are often immune to Ice, and are always immune to the Poison qualifier. Some undead require special weapons to affect; for example, silver weapons will often inflict full damage where normal weapons might not.
Undead are healed by the Necromancy effect group and harmed by the Earth effect group. Anything that specifically targets undead (such as Turn Undead or Destroy Undead) will work against these creatures. A Life spell will inflict a Destroy Undead effect against an undead.
Undead generally have large amounts of Natural Armor, representing the powerful necromantic energies that animate them holding firm against any attacks. Healing ignores this armor, making it a powerful weapon against the undead. Additionally, many undead are limited in how they can use defensive abilities against Earth effect group attacks.