A ghoul is a supernatural creature that eat corpses.  First mentioned in Hounded as associates of Leif Helgarson, they are largely unseen characters in the novels, only appearing in the short story The Demon Barker of Wheat Street.  A group them (called a "shroud") operate in Tempe, AZ under the leadership of one named Antoine.  Leif keeps them on speed dial in his phone to help him with body disposal and crime scene clean-up, especially in situations where the supernatural was involved.  They use a refrigerated truck to remove and store bodies they cannot eat right away.

Unlike most fantasy works, the ghouls of the Iron Druid universe are not undead, but living human beings with a genetically inherited condition.  Their origin is not explained until The Demon Barker of Wheat Street.  Atticus narrates that all ghouls are descendants of a single man in 2nd century Arabia.  A wizard used the man's body as a vessel to summon a demon, which also had a taste for human flesh.  The wizard provided the possessed man with corpses to consume, which not only pleased the demon but gave him incredible strength.  Eventually the wizard realized he'd made a terrible mistake by creating this deal and exorcised the demon from the man's body, planned to kill him as well to destroy the evidence of what he had done.  However, the demon's presence had altered the man permanently, making him inhumanly strong and still with a desire to eat dead flesh.  The man killed the wizard and fled, eventually learning that his skin would turn gray and he would start to appear more like a corpse himself if he did not continue to eat dead flesh.  When he married and had children, these traits were passed on to them and all his decendants. 

A ghoul is born a completely normal human and nothing about them is unusual until they reach puberty, when their skin begins to gray.  At that time, the parent must explain their nature to them.  A ghoul who is well-fed appears to be a normal human, but has enhanced strength.