Demon is one of the three classifications of monsters in Diablo II. All monsters belong to one of the classifications in Diablo II, and certain spells or items are more effective against different types of monsters.
It's not known if these classifications will return in Diablo III, but we're using it since it's a convenient way to sort things, until official information is released.
The three classifications of monsters we use in Diablo III are:
- Animals - Feral, unintelligent enemies.
- Demons - Mostly Sinister, sentient monsters from Hell.
- Objects - Stationary obstacles, like traps.
- Undead - Varying types of re-animated corpses, mostly from humans.
For a full list of Demon monsters, check out the Demons Category. You can also go to the Diablo 2 Wiki to the find Diablo 2 Demons.
The Demon classification is for monsters originating from hell, or that is so infused with Darkness that they have ceased to be their "earthly" form, and truly become more at home in the Burning Hells than on the mortal realm of Sanctuary.
Demon monsters usually have a higher level of intelligence than Animals, and most of the Undead, but can still be just one in the line of canon fodder together with animals. Many demons act officers over lesser creations, and the Prime Evils are the greatest of them all.