Demons are spirit beings who do not have a body of flesh and blood. Since demons are fallen angels, like angels they are immaterial and not usually subject to human visibility or our other senses.
They are dwellers in an unseen world. To manifest themselves among men, they must possess or control a physical body of a person or an animal. As they possessed people, their influence had its effect in several different areas.
In the Dark Ages and Middle Ages, demons were thought able to travel on the earth invisibly, but to assume physical bodies at will. The same was understood concerning angels. This has a certain logic. In order to tempt human beings, demons need to remain invisible while they whisper seductive words or cause opportunities to commit sin to fall across the paths of their intended victims. But in order to murder their victims once their temptations succeed, demons need to have teeth and talons capable of ripping flesh.
The modern view of demons, among those who believe demons in the Christian sense to exist, is that demons are usually invisible but are capable of revealing their forms at their pleasure.
It is not so widely accepted that demons can make their forms material. The modern demon is thought to punish those who give in to its temptations by possessing the victim's body and using it against the victim and the loved ones of the victim. Frequently the possessing demon causes the victim to commit a horrible crime, such as the mass murder of elementary school children, before committing suicide.