Ghouls are ugly creatures that resembles a slimy, buck-toothed ogre. They tend to live in the attics or barns of wizards and witches. They are relatively harmless creatures and are just seen as nuisances because of the noise they make. They are relatively dimwitted, and live off of bugs and other household pests. At most, they will groan and throw objects.
A Ghoul Task Force exists at the Ministry of Magic, under the Department for the Regulation and Control of Magical Creatures. The Task Force is used to remove unwanted ghouls from houses that have passed in Muggle ownership. There is a variety of ghoul, called the Chameleon Ghoul.