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.
Powers & Abilities
Their abilities and origins vary between various works (see below for more information/examples), but the most common traits of zombies are:
An unending hunger for human flesh or, more stereotypically, brains.
Appearing in large numbers (often referred to as "hordes"), and able to turn other humans into zombies via biting, scratching, or some other means.
Inability to feel anything, including pain, and thus able to shrug off traumatic wounds from blades, bullets, fire, and other hazards.
Being mindless, unable to think, learn, or feel emotions, and relying on base instincts and whatever senses they have. They are rarely able to use weapons or tools and cannot operate even simple devices such as doorknobs, and thus mainly use brute force and sheer numbers to bypass obstacles.
Often the only vocalization they make is a guttural groaning or moaning noises.
Can only be killed by decapitation or destroying the brain- otherwise, they cannot be killed by anything that would normally kill a human: they do not require air, food, water, or sleep, can stay underwater indefinitely, and can move even after losing large chunks of their body (so long as their head is still attached).