Anyone who has a powerful and, sometimes, uncontrollable desire or yearning for something. May be used in the same nature as "slut", "whore", "fiend", or "junkie", in the sense of someone who is crazy for something. However, usually "ghoul" labels the people with the more serious cases of lusting for something.
Common "ghoul" focuses are: sex, drugs, food, porn, alcohol, money, gambling, caffeine, spending, candy, etc...
"Look at that kid sucking down those chocolates. He is a candy ghoul."
"Beer...beer...BEER!" (think: Brains...Brains!)