Hungry Ghosts (Pretas) are pitable things. They are wasted creatures with huge, empty stomachs. Their necks are too thin to allow food to pass. So, they are constantly hungry.
Greed and jealousy lead to rebirth as a Hungry Ghost. The Hungry Ghost Realm often, but not always, is depicted between the Asura Realm and the Hell Realm. It is thought the karma of their lives was not quite bad enough for a rebirth in the Hell Realm but not good enough for the Asura Realm.
Psychologically, Hungry Ghosts are associated with addictions, compulsions and obsessions. People who have everything but always want more may be Hungry Ghosts.