exist in a wide variety. In general, they are horrifying creatures from other planes of existence. They range from some of the denizens of Hell to "ordinary" dimensional travelers.
The drum, bass, guitar, music, noise archetypes existing in the Vurt.
In the Shannara series by Terry Brooks, there are several creatures known as Demons. They call themselves Jarka Ruus, which means 'banished peoples'. These creatures were long ago sealed within the Forbidding, a realm of darkness where only the cursed dwell.
In Darren Shans The Demonata series, demons play an important part. There is a demon in every book.
Demons, in the Artemis Fowl Series, are the Eighth Family of Faries. During the battle of Tailtte ten thousand years ago, they fought with the other fairy familes against humans, but realised that they could never win. As a result, the warlock demons used magic to try to move Hybras, their island, but ended up sending it into an unstable Limbo.