A lich is a type of undead creature usually depicted with a body that is cadaverous, desiccated, or completely skeletal. Unlike mindless zombies, a lich retains its intelligence and personality from when it was alive and often holds power over hordes of lesser undead.
Liches are usually created through a transformation most often by a magician/necromancer to achieve eternal life or immortality. In some works such a transformation is achieved by channeling ones soul into a phylactery/item. The transformation can be stopped or the lich made mortal by destroying it's phylactery/item.