Mrs. Drull is a ghoul. She is - or was - one of the regular attendees that Windle Poons met at Reg Shoe's Fresh Start Club in Ankh-Morpork during the events described in Reaper Man. She didn't really feature in the book other than as a background character, but she gave Lupine the Werewolf a chance to advise Windle not to try one of her meat pasties:
'Oh dear,' said Windle, 'she doesn't make them out of humans does she?'
'What? Oh no - she's just not a very good cook!'
Retired now, she does childrens' party catering.
Ghouls aren't very common in the Discworld canon, but there are whispers and rumours here and there. For instance, Biers, the seminal drinking den for the differently alive, is known as a place where the ghouls can get a decent meat pasty and chips. (No questions, right? Just don't ask. Perhaps they're Mrs. Drull's.) When Carrot first proposed that a forensics division be included in the revitalized City Watch, he mentioned the possibility of hiring a ghoul to work as medical examiner; as no such individual is seen helping Cheery and Igor in later books, it appears that none were willing to accept the job, probably because the position would disallow their eating the evidence.
Ghouls, in their traditional sense of unclean humanoid things that live under the earth and hold unholy feasts in graveyards (from their point of view, a convenient source of well-matured protein) are best avoided in the works of H.P. Lovecraft and by his account live in the darker corners of old Massachusetts. Don't go near Arkham, Mass., on a moonless night and you should be untroubled by them.