Demons are essentially evil spirits, or evil angels. They have been given a degree of freedom along with Satan, and according to Scripture they occupy "the air" (earth's atmosphere) with Satan as "the prince of the power of the air". But they also take possession of humans and animals when it suits their purposes. It is very significant to note that while the Lord Jesus Christ was on earth, demonic activity increased sharply, and many people were demon-possessed. That does not mean that demonic possession was unique to that time.
It would appear that evil spirits have been possessing men ever since the time of the Flood and even after that. When we hear of bizarre, horrendous and atrocious crimes being committed today, we should stop and ask ourselves whether or not they resulted from demon-possession. But occultism and shamanism have been around for several thousands years, and have involved communication with, and possession by, evil spirits. Witchcraft is mentioned in both the Old and New Testaments, but since the introduction of New Age beliefs, it has increased greatly in Western societies. Now they prefer to call it "Wicca" and put a veneer of innocence on it. Animism (the natural world -- animals, plant, rivers, mountains, etc. -- is possessed by spirits) is a belief that exists througout primitive societies, even to this day.
We should also ask ourselves whether all these Jihadists and Radical Islamists are not demon-possessed, given what they have been doing to innocents. I would not be surprised if it was discovered that Radovan Karadzic, Ratko Mladic, Ayatollah Khomeini, Pol Pot, Mao, Hilter, and Stalin (and many others who have committed unspeakable butchery and genocide) were all possessed by the Devil. There is no other explanation for such pure evil and brutality.