Demons are either Angels who rebelled against Heaven or the Hellborn offspring of such entities. Demons who have been expelled from Hell or who were denied entry there after Lucifer's exodus are known as Stranded demons.
There is no typical demonic body type. The Fallen, angels who joined Lucifer when he seceded from Heaven, all chose to use Hell's dark materials to transform themselves in their own ways. Their offspring were corrupted by the dark energies of the Infernal Plane, and each successive generation is more twisted than the first. Once Outsiders started to settle on the Infernal Plane, they interbred with demons to create even more grotesque monstrosities. Many demons can shapeshift and cannot be rightly said to have any primary form at all. If one were to generalize, however, it could be said that most Infernal demons are larger than humans and terrifying to look upon. They generally smell of sulphur dioxide and have strong leanings toward cruelty.
Stranded demons have a more uniform appearance, being mostly small and squat with grey skin and huge, lamplike eyes. They tend to form castes, with the lowest and least powerful Stranded being ghouls and the most powerful being kings or controllers. Exceptions to the normal Stranded demon physiology include hoppers, who have been mutated into insectile forms by the influence of a controller.