A demon (from Greek δαίμων daimōn, meaning "spirit") is usually considered any entity which is generally considered negative in nature. Nearly every religion and/or magical tradition believes in some kind of demonic entity. Generally demons are considered non-human, though this may vary.
Demons are often quoted as having a number of abilities shared between belief systems. They're generally considered more powerful than normal entities, and thus more capable of manipulating the physical plane.
Demonic possession can refer to either the entity haunting a particular locations or being attached to an individual. Individual possessions can range between having one's mood and behavior affected negatively to the entity outright controlling the individual.
Demonic presences are often associated with poltergeist activity, particularly when it harms people in some way.
Shadowmen are usually associated with demonic presences, especially when accompanied by a heavy sense of dread or foreboding.
Disease and even death often follow reports of demonic residences.
Exorcism is the process of getting rid of an unwanted spirit. With demons, there are a number of suggested techniques, ranging from reciting lines from scripture (of one's closest religion), burning sage, and even simply leaving the haunted area. Possessed individuals are usually taken to an exorcist.