Ghouls are humans who have been exposed to an extremely high amount of Radiation. They still act and think like humans, but the mass amounts of radiation poisoning has caused there skin to melt and rot. Despite their appearance being the only difference, they are still looked down upon by most of humanity, often being forced to live underground or in the shadows.
The high amounts of radiation have made all ghouls completely sterile, meaning that nobody is born as a ghoul, they are exposed to radiation at a later stage in life.
Sometimes a ghouls brain is greatly affected by the radiation causing it to deteriorate, when this happens the ghoul becomes a mindless, aggressive, zombie-like creature known as a Feral Ghoul.
They have appeared in all of the Fallout games.