Zombies, the horrid, flesh eating, terrifying creatures of Vodou magic and biological warfare. Shrouded in misconception and mystery. Let's blow some of that away!!
Now in Haitian Vodou, there are two types of Zombies, one you are well aware of and that is the physical plane Zombie. Now, to clarify a zombie is not the undead. Tho they are retrieved from graveyards where they were buried, they have not been ressurected, but rather poisoned. Poisoned with a concotion engendering a state of near-death, containing herbs, liquids, and the paralyzing poison of some nasty Puffer fish. The concotion is given to a living person who falls ill and quickly "dies". Now, keep in mind, there are very few people to confirm the death in Haiti, as there are few medical professionals, and the equipment and time just aren't there for most. They follow the normal machinations of burial and ritual. The poisoner will then return and dig up the body feeding the person a curing, honey-like solution. Now, this not only cures them, but clouds the mind and puts the victim in a trance-like state opening them to obeying their new "master". Now, generally, they will remember their name and how to eat. Beyond that, they are a little fuzzy. They won't be sent out to eat, attack, or maim people. They are used as a slave like labor force. There isn't much magic to all this! They are not enslaved spirits or any such nonsense. They are drug addled persons. In fact, the fear of such things affects some in such a way that the head is chopped clean off as part of the post-mortem rights, someone will watch over the grave, and in some cases large stones will be placed to prevent the disinterment of the dead.
The second Zombi, or Zonbi are also known as mo, is of the astral or spiritual variety and can be far more burdensome on a person. They are sent in a practice called Anva Mo, which translates to a sending of the dead. This attack isn't subtle and can be like a minor possession depending on the severity of the attack. Illness, terrible luck, voices, etc. will manifest. The attacked will know they are being attacked, but will generally see themselves as going crazy or become too paranoid to seek help. Any number of mo can be attached to a person and the mo will do their best to take the life of ther person until the person dies, the sender of the mo calls them off, or another person (generally a Mambo or Houngan) relieve the person in a ceremony also called Anva Mo. The ritual will cleanse the person of the Mo and send them right back to the orignal sender (much of Vodou philosophy is surrounded around defense is the best offense). This is done in an elaborate ritual uner the hands of a Mambo or Houngan. Done in the graveyard for the Baron of the graveyard, the person is chanted over and cleansed until the Mambo or Houngan has determined that the Mo are gone. It can take days for the person, depending on how they had been carrying the Mo, to recuperate or even walk again.
They are spirits of the dead that already exist. I can't say much on the actual ritual as it is an initiatory secret, so it's not my place and I don't actually know much more beyond what I have shared.