A zombie is a dead person that has been brought back to life by external (i.e., supernatural, scientific, or other) forces. Generally, the term refers to a person who is revived after burial by voodoo (or other magic) and compelled to do the bidding of the reviver, including criminal acts and heavy manual labor. Other times, it refers to ghoulish, undead persons with a single-minded and obsessive desire to hunt and kill the living and devour their flesh.
Night of the Living Dead
Zombies were reanimated under unclear circumstances (theories by the human population included radiation and an overflow of souls from Hell). These zombies, slow and dull-witted, sought to devour the flesh of the living. A bite from one of these zombies mean that the victim would eventually join the undead. A shot to the brain would kill these zombies. (These zombies were incorrectly referred to as ghouls in their initial appearance.)
Dawn of the Dead (Remake)
The zombies were much faster, if otherwise the same.
The zombies were similar to the Night of the Living Dead version, except that they were reanimated through unknown means (possibly voodoo, due to the South Seas setting).
Return of the Living Dead
The zombies were much tougher, reanimated by a chemical known as Trioxin 245 which made them hunger for human brains (to ease the pain of being dead) and much quicker in reflexes and intellect than the Night zombies. Not even a shot to the brain would stop them, and incinerating them would simply create airborne trioxin that could reanimate more dead. It was later discovered that they were vulnerable to electricity.
Later, an apparently slightly different chemical, Trioxyn-5, created zombies with the speed and intellect seen previously, but with the vulnerability to brain damage seen in the Night version. They also appeared to be vulnerable to simply being shot a lot.
Shaun of the Dead
Zombies were slow-moving, uncommunicative, aggressive, and generally difficult to distinguish from the less dead inhabitants of London. They ate human flesh, and a bite would result in zombification within a few hours. They could be destroyed by significant damage to the head or brain.
Numerous theories were suggested for the creation of the zombies, almost all of them in reference to other zombie movies. These included: infection from outer space, radioactivity, military experiments, and rage-infected monkeys.
After the crisis, the remaining zombies were put to productive use in menial labour and TV game shows.
Dungeons and Dragons
These zombies are magically animated corpses, which can be easily dispatched by inflicting sufficient damage. There are several specialized variants, however, including the ju-ju zombies originally of Oerth, the Strahd zombies and zombie lords of Ravenloft, and many others found in further worlds (such as Toril, Krynn and Mystara).
The zombies were animated by the T-Virus. Their desire to consume human flesh was caused by the remaining primal need to eat. Like the Night version, a bite spread the infection, and a shot to the brain killed them permanently.