There are three major types of zombie film; each one was once popular but in general gave way to more exciting and epic genres of zombie. In order:
Necromancer zombie films: A necromancer raises up some bodies for his own purposes, giving them a semblance of life, one by one. Only a few modern zombie films follow this "voodoo" stereotype, and include the Reanimator films. See: Necromancers and Zombiefication: Toxins and Voodoo.
Apocalyptic/epic zombie films: Since George Romero's Night of the Living Dead (1968) a new form of zombie film created a new genre of film. The recently deceased started re-animating and attacking the living. The zombies are slow and suffer from rigor mortis, and move awkwardly and slowly. Their strength is solely in numbers, plus an uncanny ability to tear apart the bodies of the living with their hands and teeth. The phenomenon tends to spread and become more serious over time, often causing the collapse of civilisation and threatening all of humanity to extinction.
Fast and aggressive zombie films: 28 Days Later (2002) saw a new breed of zombie arise which is fast, aggressive and strong. These films have proven more popular and more exciting than any of their introspective predecessors, World War Z (2013) being particularly successful.
Of the slow-zombie films, two long-standing series defined the genre, and made all future films possible. They are:
George Romero's of the Dead zombie films are by far the most important and 'purist' of the zombie films. They started with Night of the Living Dead, Dawn of the Dead and Day of the Dead which documented the rise of the zombie plague from a nuisance to dominance. The third film was very dark indeed, with survivors existing only underground, in increasingly desperate conditions. This series is sombre, progressive and strife-orientated, with a tendency towards classy b-movie style. These were then followed by Land of the Dead (2005), Diary of the Dead (2008) and Survival of the Dead (2009).
Dan O'Bannon's Return of the Living Dead series was a comical and fun progression from Romero's Night of the Living Dead, and zombies follow all the same rules. The main difference is that the zombies are more talkative.
Commentary of Romeo's films can be found on Classic Zombie Films: the Slow Undead. Also see The Best Aggressive Fast Zombie Films and a separate page for the hugely successful World War Z (2013) Film Review.
Zombie films fall into two major types:
The Build-Up zombie film often starts with just one zombie, or just one small town in the middle of nowhere. This one zombie encounters the cast of the film, and eventually turns more and more people into zombies. The film expands, with state-wide or nation-wide news reports showing us that this is a growing epidemic of increasingly serious proportions. During the build-up, the cast have to fight harder and harder to stay alive, and eventually are forced to find somewhere safe and secure, where they barricade themselves in against the walking dead outside. The second half of the film then follows the second major plot: The Survivors.
The Survivors zombie film plot has a group of survivors in a secure, fortified location trying to live their lives. Largely, the zombies are kept at bay. Existential questions are asked: What is the meaning of life, what can we do in the long term, can we continue to live like this? Etc. The plot always develops that internal personality clashes or politics leads to the group breaking apart, and the result is that the security of the compound is compromised, and the zombies get in. This is frequently due to the reckless action of a dissenter in the group, who opens the doors or who tries to leave.
Mysticism and the Unexplained: Not all zombie films offer explanations of why the dead now refuse to die. Some films just drop esoteric hints, such as the subtitle to Dawn of the Dead: "When hell is full, the dead will walk the Earth". This implies that the living who die are largely going to hell, but, at some point hell is full, so the souls of the dead linger on, animating their dead bodies. The living pay for the sins of their (dead) compatriots, as society as a whole has failed.
Radiation: In Night of the Living Dead the explanation is a Faustian rejection of science: Generic 'radiation' from a 'space probe' that exploded is 'increasing' throughout the film (as heard on background news reports), and this may be causing the recently dead to return to life.
A Virus or Disease: Many zombie films explain the morbid happenings as the result of a disease: Some kind of spreading infection, if caught, causes a severe cold that results in death, and the dead then return to life. It is like a possession where the 'original' person is slowly replaced by a zombie, and the final transformation occurs when they die. This can be seen in Romero's zombie films (except the first one) such as Dawn of the Dead and Day of the Dead, and in others such as 28 Days Later, World War Z and Shaun of the Dead. The infection spreads from its original locale across a state, a country, or even across the world.
There is normally a generic rejection-of-science theme in zombie films, where Humankind itself causes the apocalypse by tinkering with new scientific technology or bio-engineering. This is the case in Night of the Living Dead where the outbreak is attributed to radiation brought back to earth by a space probe and in 28 Days Later to a genetically modified virus and the Return of the Living Dead series blames an evaporated poison. Only some more outlandish zombie films break from the anti-science trend, for example Braindead which attributes the disease to a natural virus resulting from a dangerous hybrid rat-monkey.
Real Theories of Real Zombies:
Fungal Infections and Parasites: In the real world the closest thing we find that comes close to zombiedom are the results of parasites such as fungal infections and parasitic insects, rather than viruses or bacteria. These spread by sporing, dripping or hatching their way from victim to victim. There are some viral zombie-like parasites, such the baculovirus variant that infects gypsy moth caterpillars, making them climb high up into trees, before turning to mush and dripping infectious gloop on to other caterpillars. See NBC News blog entry for an exaggerated list of natural 'zombies' (dated 2013 Jun 22).
Tetrodoxin (TTX) is sometimes cited as a possible cause of human behaviour that is zombie-like, however, these claims have been thoroughly discredit, and the poison cannot actually fulfil this role. See: Necromancers and Zombiefication: Toxins and Voodoo: 2. Can Tetrodoxin (TTX) Create Real-Life Zombies?.
Most zombie films have the zombies hard-to-kill unless you know how. Their bodies are weak and easily damaged... but, normal damage does not kill them. They feel no pain and do not care to preserve themselves or defend themselves. Their only motivation is to attack the living. To kill zombies, in a classical zombie film, you have to:
Destroy their brain. If you destroy the nerve centres of the brain, especially the modules that control motor functions or the brain stem, you disable the zombie. Cutting its head off means that it can't control its body, and the body becomes a normal dead body.
Burn them. A burning zombie will normally become distressed, more confused than normal, and will sometimes try to pat itself out (though it can never succeed). After a while, the zombie will collapse. Fire will frequently scare zombies and keep them at bay.
Demolish them by crippling and destroying their whole body. Once a zombie is shot in the head, flaming pyres are the best place to dispose of the body. Some films have it that this destroys the virus too, otherwise the corpses can still spread infection.
There are exceptions... in 28 Days Later, zombies have more normal bodies and can be killed like a normal person. In Braindead it appears that the zombies entire nervous system is given a murderous will to kill the living: In the most strong-willed zombies, their body parts remain animated even after destruction of the head or after truncation from the torso. And in Reanimator, the injected liquid appears to give any flesh an individual will and mind. But these films stray from the zombie 'norm' of brain-based motivation, and therefore death by brain death.
Where do zombies' motivations and willpower come from? There are two general trends in zombie films:
Materialism: The brain is the source of willpower, so, a biological disease, radiation or a chemical makes changes in a person's brain, so that when they die certain parts of the brain remain active and motivate the zombie. Zombie willpower is fundamentally a result of a broken brain. This is seen in most apocalyptic zombie films and some necromancer ones.
Animism: Motivation comes from a more mystical, ethereal source. Zombies' wills are fundamentally an evil, psychic force from beyond the grave. It is animist in that in certain films containing zombies (Braindead and the Reanimator necromancer films), body parts can obtain wills of their own. This clearly disassociates willpower from coming from the brain.
Animism is unbelievable for one major reason:
“Motor functions in our nervous system are controlled centrally by organs at the base of our brain, at the top of the spine, such as the medulla and cerebellum. Orders travel from here, down the spine, to the appendages. The appendages' nerves have no system to self-rule. In other words, muscles can only be controlled in a directed way, from the brain. As a result of this, animist zombie films must somehow explain that (for example), a hand on its own has had its nerves physically rewired into a new motor control area. It becomes far too complicated and unlikely that mystical sources of willpower could ever accomplish this.”