Ghouls are creatures from legend who feed on the dead. They inhabit crypts, cemeteries, burial mounds and battlefields. It is said these nocturnal abominations are harmed only by fire, silver and bright light.
The Ghoul comes from Arabian folklore as a demon who could shape shift into any animal, and most often a hyena. The creature would be known for preying on children, stealing coins, and taking the form of the most recently eaten human. The oldest surviving literature that mentions ghouls is considered One Thousand and One (Arabian) Nights.
The depictions of ghouls has varied greatly over the years, ranging from macabre but harmless carrion scavengers, to feral inhuman creatures, to nigh all-powerful demonic monstrosities. The following is a basic classification of some of the notable ghoul varieties.
Essentially the ghul as depicted in Arabic lore: demonic flesh-eating, shape-shifting jinn that inhabit graveyards. This is one of the least common depictions of ghouls however.
In the writing of H. P. Lovecraft, most notably Pickman's Model, ghouls are a vaguely humanoid, nocturnal subterranean race. Humans can mutate into one of these ghouls from a diet of human corpses and further aid from proper ghouls. Lovecraft's ghouls are usually described as white or green-skinned hairless creatures with long canine muzzles, pointed ears, and clawed feet that have nearly become like hooves. They inhabit networks of underground tunnels and crypts, and eat corpses.
Basically just zombies by another name, or a specific variety of zombie. Zombie ghouls will generally be more feral than regular zombies, as well as more willing to devour rotten flesh instead of only living.
These ghouls are either created by vampires to act as servants, or they may be a related offshoot of standard vampires. They vary from being immortal - albeit twisted - human beings to mindless zombie minions, to beings that are actually more powerful than vampires themselves. The Hellsing manga and anime for instance, depict ghouls as humans who had their blood drained by vampires and were subsequently transformed into zombie-like slaves.
Former humans that were transformed into ravenous zombie-like beings by either a virus, radiation, or some form of malevolent supernatural entity.
In the popular Fallout video game series for instance, ghouls are the human victims of heavy radiation poisoning, causing their bodies to decay, while paradoxically extending their lifetimes considerably. Feral ghouls are a common enemy in most of the games, but there are also those that retain their intellect and can still behave like normal humans despite their undead state.
They're similar to mutant ghouls, but were transformed by magic or divine punishment rather than a virus or radiation. They are typically people who are punished for inhuman acts such as greed, murder, or especially cannibalism. These former humans are still alive, but have been turned into flesh-eating monsters that tend to haunt graveyards. They often grow razor-sharp claws, fangs, long limbs, and lots of hair - making them similar to a Wendigo.