Ghoul

Ghouls are decrepit, ragged, almost rotting, zombie-like mutants, victims of massive radiation poisoning.

Origins
Most known ghouls in the Core Region were created from vault dwellers living in Vault 12 under the city of Bakersfield (better known as the Necropolis after the Great War). As part of the vast Vault Experiment Program, the Vault 12 door was designed not to close properly. Thus, massive amounts of radiation attacked those within the Vault, most of whom were turned into the current ghoul population. Generally, in the Fallout universe, massive exposure to radiation causes humans to transform into ghouls.

Biology
All ghouls are completely sterile. There is only one generation of ghouls in the wasteland and it is the last. Furthermore, the ghoul transformation grants its subjects extremely long life. Ghouls created in the Great War of 2077 were still alive during Fallout (2161) and Fallout 2 (2241). Ghouls are generally as intelligent as normal human (more or less, although not just as charismatic, you might as well say). However, their repulsiveness makes the life of a ghoul difficult at best. Ghouls are, naturally, immune to radiation. Radiation poisoning can't really get worse for them.

Variations
Some ghouls absorbed so much radiation that they glow in the dark. They are called the Glowing Ones and are often considered outsiders even by other ghouls. The Glowing Ones also emit radiation, so they can be dangerous to normal humans.

Harold and Talius, even though they look very much like them, are not ghouls per se. They are of a special type of mutant, created by brief exposure to FEV, the so-called "FEV ghouls".

Through cruel experimentation on humans by Dr. Sebastian at the Reservation, Born Ghouls were created. They are ghouls who were not mutated smoothskins, but who were actually born into ghoul-dom.

Appearances in games
Ghouls have appeared in every Fallout game so far, although there has been no mention of them in any of the Fallout 3 previews.

Trivia
A ghoul is described as "Bloodman" in an old Fallout concept art.

Quotes

 * There ain't any ghouls but old ghouls. We're all sterile, see, but we're incredibly long-lived. We're the first and last generation of ghouls. - Typhon
 * Without medical technology, all of us Ghouls are going to die off in the next 20 years, anyway. - Gordon (Gordon dialogue file)
 * [Say, how do you make a ghoul?] With silver-bells and cockleshells and… Boy, you are dumb, aren’t you? Severe radiation. That’s how. How do you think? You know, many bombs go boom, flash of light and heat, flesh burns off, but you don’t-quite-die-type severe radiation? - Wooz (Wooz dialogue file)

Controversy
There is some controversy even among the makers of Fallout games about the origins of ghouls. While Tim Cain said explicitly that ghouls are only a result of radiation, consistently with the "SCIENCE!" of the 1950s, Chris Taylor said that a mix of both radiation and FEV was in play (which would probably mean that there aren't any ghouls outside the Core Region). While Chris Avellone initially supported the latter view in his Fallout Bible, he was later convinced to support the radiation-only version.