In the ancient cultures, a demon didn’t always have an evil connotation. They were simply beings of spiritual nature, non-mortals. However as religion evolved so too did the demons.
The Bible refers to them as fallen angels. Spirit rebels against god that have the ability to take possession of a physical body. Demonic possession is an event of complete takeover of a person by a spirit. Demons are something people feel, unlike monsters, which one may have a chance to actually see.
Be forewarned! Do not to speak the name of a demon out loud. Doing so may enhance its power and summon it to you.