Ghoul

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

Origins
Most known ghouls 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 aswell say). However, their extreme ugliness and physical frailty makes the life of a ghoul difficult at best. Ghouls are, naturally, immune to radiation. Radiation poisoning can't really get worse for them.

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.

Harold, even though he looks very much like one, is not a ghoul. He is a unique type of mutant, created by brief exposure to FEV.

Appearances in games
Ghouls have appeared in every Fallout game so far.