There is as much lore about zombies as there are walking corpses in Night of the Living Dead, much of it contradictory.
Zombies are basicaly animated corpses. That is the only point the various stories agree on. In some versions they are mindless automatons, in others they are as intelligent and self-willed as they were when alive, only missing soul and concience. There are many different ways to create zombies, and just as many reputed ways to destroy them. Most sources agree, however, that zombies must be created. They do not return on thier own, like ghosts, and they are not created by a contagious curse like vampires (except in B horror flicks).
They are raised by a spell or curse, though the methods vary from natural necromantic talent to hoodoo Corpse Powder.
Silver, fire, severing the head from the body, and nailing the corpse back into its grave bed are just a few of the many supposed ways of destroying them.
Comments
Soulless
So I suppose the reason they'd have no conscience is because when they died their soul's left their bodies and then when they were reanimated they were soulless, hence no conscience? Does that make sense?
Yep!
That's it exactly.  The person's soul or spirit (however you want to put it) has moved on to the next life and all you have is an empty shell, maybe some memories (or partial memories) and a rudamery power of speech (depeding on the type we're talking about).    That's why it takes great (and mostly dark) powers to sucsessfully and fully raise the dead, otherwise you end up with a zombie.
Makes sense.
It does make sense. That was pretty much my theory, too. It seems like when the soul passes on to wherever it's suposed to be it's nearly impossible to bring back, which is why the vampires in the Buffyverse always made sense to me, being demons inhabiting the body and sharing the memories of the person who was turned, rather than the person themselves.