From D&D Wiki
Unliving creatures are undeads that mimic many of the capacities of a living creatures without truly being alive. They don't have an actual metabolism (although they have some parallel system which allow them to feel much alive than any other undead), and medical check in dc 10 will expose the unliving's undead nature. Unless noted otherwise, unlivings are completely immune to turning and similar effects. Unlivings can be either improved versions of undeads, something which exist between undeads and living creatures or even an evolutionary biological anomaly- doesn't matter what they actually are, they are not alive in the standard way, but are far from being just walking corpses or remains of broken souls; they are still, and always will, own their souls, and are more alive than most undeads can even imagine (or remember they ever were).