Ghoul

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

Origins
Generally, in the Fallout universe, limited exposure to radiation for a longer period of time causes humans to transform into ghouls, although rare cases of faster transformations into ghouls are also not unheard of.

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 either died or mutated, eventually turning into ghouls.

Ghouls from other regions most likely originate from other, similarly ill-fated shelters that shielded their residents from radiation enough for it not to kill them, but not enough for them not to mutate.

Biology
All ghouls are completely sterile. There is only one generation of ghouls in the wasteland and it is the last. Ghouls created in the Great War of 2077 were still alive during Fallout (2161) and Fallout 2 (2241). Some ghouls live longer than normal humans, while the majority do not. It has to do with cellular level, and the ability for some ghoul DNA to make more copies of themselves. Basically, cells only contain enough material to be duplicated X number of times. In a ghoul, sometimes, additional genetic material is added.

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 - only the most tolerant human communities accept them as anything more than monsters. Some ghouls eventually go mad and are called "feral ghouls", giving all ghouls bad reputation.

Ghouls are, naturally, immune to radiation. Radiation poisoning can't really get worse for the ghouls. In fact, ghouls now thrive in radiation, making their homes near sources of acute background radiation. The "Glowing Ones" actually enjoy large amounts of radiation, which they describe as being "comfortably warm".

Feral ghouls
Feral ghouls are those of the unfortunate mutants that lost their minds and became aggressive animals. Their intelligence is almost completely gone and they react mostly on instinct.

Glowing ones
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.

FEV ghouls
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".

Born 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.

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 , which is also confirmed in Fallout 3.