Zombies are corpses created with grim spirit magic. The soul is pulled from the spirit plane, and forced to inhabit its former, unliving body. The spirit is bound by enchantments, making it impossible to rebel against its creator.
As a result, the soul is unable to go on to its final reward, and in fact is witness to every deed it may be forced to perform, helpless to do anything.
If a zombie is fed salt or meat, the ritual's effects are dispelled. Note, that while a zombie may be directed to tear flesh with its teeth, this doesn't involve actual eating. Many times, the mouths are sewn shut to make this less likely to happen.
The magic used to animate a zombie stops it from rotting, so a freshly made corpse may very well seem alive at first glance.