Zombies are fictional beings created through the reanimation of human corpses. They are not really alive, but cannot exactly say they are dead either, hence the alias undead. Zombies are most commonly found in horror and fantasy genre works.
Zombies, in folklore, is a concept that is rooted in the African religion of Voodoo - according to legend, zombies are reanimated corpses that has been bestowed life by a bokor, a practitioner of black magic, once reanimated the Zombies are slaves to the bokor and have no will of their own: existing to do as the bokor commands. So under the command of evil and malevolent bokors Zombies can be dangerous.
The Zombie of folklore bear little resemblance to their Hollywood counterparts and do not crave human flesh, nor are they often depicted as rotting and decayed - in most accounts a Zombie in folklore is devoid of all emotion, slow-moving and drained of life yet could be mistaken for someone who is gravely ill rather than a walking corpse, often a Zombie in folklore is only revealed as such when a community or relative identifies the person as having been deceased.
Although seen as a fabrication by most mainstream scientists their has been a controversial theory that the folkloric Zombie described above did exist (and may continue to do so) - however they were not walking dead, rather the theory has it that the bokor utilize advanced poison (perhaps from blowfish) to induce a death-like state in their victim that lasts for several days, sufficient time for grieving relatives to bury them and the bokor to exhume the grave and release their "walking dead".
This theory is not accepted by many scientists and regardless of the truth there are still places in the world where stories of Zombies can be found and the fear of encountering (or worse becoming) the "walking dead" is still very real. For this reason, bokor are afforded a great deal of respect.
Modern depictions of zombies often invoke science fictional means such as carriers, radiation, diseases, vectors, pathogens, scientific accidents, etc. Modern zombies are usually crazy for human flesh, although Return of the Living Dead introduced the innovative concept of zombies eating brains. The "zombie apocalypse" concept, in which the civilized world is overrun by a global zombie infestation, became something of a staple of modern popular art.
The Serpent and the Rainbow (both the book and a movie based upon the book).
White Zombie (1932 black and white movie starring Bela Lugosi).