The ghouls are the most nefarious beings Bod comes across in the graveyard. The ghouls are ugly, dirty, and completely self interested. Their biggest interests are eating corpses, turning others (like Bod) into ghouls, and returning to Ghulheim, their ghoulish capital located somewhere in Hell.
Comically—and grotesquely—the ghouls take on the name of the first person they eat. Each of the ghouls Bod meets has a preposterously fancy name, like The Emperor of China, The Duke of Westminster, and The 33rd President of the United States. In contrast with their names, the ghouls are small, nasty creatures with sharp teeth, perfect for eating human flesh.
The ghouls are quick to manipulate Bod, convincing him that they care about him and that he should join them. They are, however, purely self-interesting. Perhaps Bod will become a ghoul himself and join their escapades. Perhaps they'll simply eat him if they get hungry. The ghouls don't particularly care, as long as Bod serves their interests.
Meeting the ghouls teaches Bod the true difference between good and evil, and that he can't trust everyone he meets. Seeing their city also makes him appreciate his home in the graveyard and all of the beings who look after him, even including Miss Lupescu, who he previously disliked.