Ghouls are mutated humans born when a human is exposed to large amounts of radiation.
Cooper Howard is referred to as “The Ghoul” possibly because he is one of the first examples of the species, but the wasteland is filled with them.
Ghouls are affected by skin and flesh deterioration but they are also resistant to radiation and do not get sick from drinking or eating infected substances.
They depend on an unidentified chemical to maintain their sanity or else they become feral after losing their mental faculties.
Despite their appearance, they are not considered “zombies” or “undead” and within the Fallout lore, they consider those terms offensive.
The many pros and cons
If a ghoul has access to the “chems”, they are essentially immortal as evidenced by Cooper’s longevity.
However, they are discriminated against by regular humans and are forced to scrap for survival.
While being close to an explosion can cause ghoulification, humans can turn into ghouls artificially as in Thaddeus’s case.
He takes a cure from the strange doctor and turns into a ghoul, which is why he has to flee from the Brotherhood.
Lucy’s mother Rose also turns into a ghoul, although her progression is too far ahead and Lucy has no choice but to free her from her misery by killing her.
Many ghouls have some form of regenerative qualities and heal from most of their wounds like Thaddeus did.
There are limits to the regeneration based on the example of Cooper having to sew back his finger after it was sliced off.