Ghoul
Ghouls are monsters that have appeared in a wide variety of forms throughout roleplaying history; if you have a fantasy or a horror setting, then you're very likely to encounter a ghoul in some form. Taking their name from "Ghul", an Arabic evil spirit or djinn that haunted graveyards and feasted on the corpses interred there, ghouls are typically undead creatures that are driven by an insatiable urge for flesh. Ardent necrovores, most forms of ghoul in /tg/ media also happily kill living beings to produce more corpses for them to feed on.
The "iconic" ghoul in most minds, based on the Dungeons & Dragons version, which is in turn based on the H.P. Lovecraft version, is actually fairly close to the iconic "Romero Zombie"; an undead flesh-eater that ceaselessly hunts for prey and which carries debilitating diseases that can cause those it wounds or kills to eventually become ghouls themselves. This is actually quite fitting, as Romero himself referred to his original movie's monsters as "Ghouls", as at the time people only knew zombies from their role in Haitian mythology as undead automatons.
All Flesh Must Be Eaten
In addition to the ability to reference Romero and call any zombies you create "ghouls", actual ghouls appear in the Atlas of the Walking Dead sourcebook. They are portrayed as intelligent, rational, unearthly beings with the ability to shapeshift between human-like and zombie-like forms, magical skills, and a need to feed on corpses.
Call of Cthulhu
As per their depiction in H.P. Lovecraft's books, Ghouls in the Cthulhuverse are a fairly benign race of alien creatures who look like someone mixed a humanoid with a dog and a carnivorous donkey and then allowed it to get slightly corpse-like. They feed exclusively on dead flesh and can transform willing humans into members of their own race, but have little malign intent towards humanity.
Probably. I mean, there're stories...
Dungeons & Dragons
The most iconic form of ghoul in many eyes, D&D ghouls are undead who can spread through an infectious disease they carry in their fangs/claws, be created by necromancers, or spontaneously arise from the bodies of cannibals or evil gluttons. They are faster, more agile and more intelligent than zombies, with at least a bestial level of intelligence, and a genuinely macabre and malevolent nature. They are perhaps most infamous for their paralyzing touch, which lets them freeze your PC immobile so they can then eat them alive; this notoriously cheap ability has frustrated many players, especially because, traditionally, elves are immune to its effects for no discernible non-fluff reason. Ghouls also have more powerful versions, such as the Ghast (which is basically a leveled-up ghoul with a permanent stinking cloud aura who can paralyze elves) and the Ghoul Lord, and aquatic versions called Lacedons.
In 5e, we are actually given a reason. You see, Ghouls came to being in the Abyss, where an Elf worshiper of Orcus turned from his people and feasted on humanoid flesh to honor his god. The Demonic Prince of Undeath, pleased with this development, made the elf into the first ghoul, and he happily created ghouls from the other servants of Orcus, until Yeenghou robbed him of his domain. He turned to Orcus, who refused to help. Thus, he turned to the elven gods, who took pity on him and helped him escape. Ever since then, elves are immune to the ghouls paralyzing touch. Still doesn't quite make sense, but it's something at least.
Warhammer Fantasy
In the Warhammer Fantasy world and the later Age of Sigmar setting, Crypt Ghouls are corrupted humans and their descendants reduced to cannibalism, something that irrevocably tainted them to the point they are not undead, but close enough thanks to Death Magic flowing through them that they instinctively obey the Vampire Counts. Most are drawn to Strigoi who are the Vampiric equivalent of themselves and make any Strigoi into their "alpha male/female", which is why Strigoi Vampires are called "Strigoi Ghoul Kings". In Age of Sigmar, Ghouls are not as rare thanks to Ushoran wandering the setting and spreading an infectious madness everywhere he goes, making members of all races and allegiances turn into packs of feral cannibals while in their mind they are lords and ladies that fight monsters and share in the spoils before attending elaborate feasts in grand halls.
A small number serve the Lahmian Bloodline, specifically a handful of tribes that worship Queen Neferata as a goddess.
Sometimes Ghouls will drink Vampire blood, usually as a gift from their master. This shortens their lifespan drastically but gives them great power, turning them into gigantic hulks called Crypt Horrors. Horrors regenerate wounds, are far stronger than any mortal being, and are immune to the magic warding of the servants of Morr and Sigmar which enables them to invade and desecrate cemeteries so their lesser kin and masters can enter and feast without fear or pain.
World of Darkness
Ghouls in the World of Darkness have no real resemblance to any of their cousins mentioned here, and are mostly named as such because "Renfields" was a stupid name for a minor template. In essence, in Vampire: The Masquerade and Vampire: The Requiem, vampires who feed a mortal regularly with their blood can give that mortal ever-lasting youth, a vampire-like healing factor, and technically minor access to vampire powers. Why would they do this? Because it also puts a mind-whammy on the ghoul compelling them to love, honor & obey the vampire with all their hearts, and with the added stick that missing a regular feeding will strip the ghoul of all its powers and cause age to rapidly catch up with them, vampires find ghouls essential as the closest things they can know to loyal, trustworthy servants capable of operating during the day.
Fallout
In the Fallout universe, some humans who are irradiated don't die horribly, but mutate; their skin sloughs off and their nails turn into claws, but they become seemingly immortal, in the "immune to aging" sense, and immune to radiation - they can even learn to "feed" on radiation to replace the need for physical sustenance. These zombie-like bastards are known in-universe as "ghouls". Many retain their human minds and now have to put up with prejudice from their former neighbors; this is not only because they look like burnt walking corpses, but because the trauma caused others to lose their minds and become zombie-like roving cannibalistic horrors, known as feral ghouls.
Shadowrun
All undead in the Sixth World are infectees of the HMHVV, or the Human Meta-Human Vampiric Virus, a magical disease that reshapes the host's DNA and aura to turn them into a monster. The first variant creates a whole rainbow of race-based vampire variants, the second a mess of lycanthropes, but the HMHVV III strain, also called the Krieger strain, always creates ghouls. It both the most virulent of the three strains, transmissible via all manner of bodily fluids and not requiring an Essence transfusion, and one of the suckiest.
Ghouls in this case are normal people who look like walking corpses, with ugly cuts and freakishly pale skin. They can go out in sunlight, though it causes them discomfort. All ghouls are rendered blind by tissue necrosis, though their senses of smell and hearing sharpen to compensate. Like all HMHVV infectees, they need to eat metahuman to survive, in their case raw flesh. Some are driven insane and turned feral by their infections, some aren't, but all need to feed on metahuman meat or starve. Those who aren't animalistic monsters tend to get jobs that offer ready access to human flesh, like medical waste disposal workers. Their bodies don't accept implants well, but they're stronger than normal, so some do work as Shadowrunners.