Demons are the Elementals of Dark. Often percieved as the ultimate evil, they are sometimes confused and associated with the Others, although this link is false. Others are pure, unadulterated hunger for magic. Demons are the essence of shadows, and through this cruel and manipulative creatures.
They will ally themselves with a Secret, however, if they need to. Demons are also the prey of the Others, so partnerships do exist, although the saying to "sell your soul to the devil" is founded in fact.
As opposed to the Angels, demons are formed from the souls of particularly evil and cruel humans, warped in Hades through exposure to dark magic. The very purpose of the Demons is to continue their existence, and the war that they fight against the forces of Heaven is a war for the amount of souls they can influence and reap.