WHAT ARE DEMONS?

A study of the origin of the word demon, indicates these entities were observed long before the ministry of Jesus. The term ‘demon' is the rendering of two Greek words which Plato derived the meaning ‘knowing' or ‘to know.' Later, an early church Father, Eusebius rejected this definition and said it instead meant ‘to fear.' The Dictionary of Deities and Demons in the Bible (Brill Publishing, 1995) says the etymology (of the word) more likely stems from the root δαίω (daio),‘to divide (destinies)'. Thus the word could designate one's ‘fate' or ‘destiny', or the spirit controlling one's fate, one's ‘genius'. Commonly the word designated the class of lesser divinities arranged below the Olympian gods, the daimones. Hesiod describes them as the souls of those who lived in the Golden Age, who now invisibly watch over human affairs.

Two verbs from this root are important in Biblical and related literature: δαιμονάω (daimonao) and δαιμονίζομαι (daimonizomai) . Both originally meant ‘to be under the power of a god or daimon', which condition was often a blessing, producing prophetic utterance or heroic behavior; it could also be a curse, and the words could mean ‘to be insane'. In later authors, especially Jewish and Christian, they came to mean ‘to be possessed by a demon' which caused bodily infirmity or insanity; in the sense ‘to be insane' it was used pejoratively of the ‘ravings' (= ‘doctrines') of heretics (Eusebius, Hist. eccl. 7.31.1 of Mani).

WHERE DO THEY COME FROM?
One ancient theory of the origin of the demons was that they were the souls of the dead who, having been unjustly treated or killed, sought retribution. Another conception was that they were the ghosts of the wicked dead (Josephus, Bel. Jud. 7.185: “demons [are] the spirits of wicked people who enter and kill the living”). Origen tells us that the Church had no clearly defined teaching on their genesis; his view was that the Devil, after becoming apostate, induced many of the angels to fall away with him; these fallen angels were the demons (De Princ. pref. 6; Tatian, Adv. Gr. 20; cf. Rev 12:4). The most popular myth, however, is found in the Bible, intertestamental literature, the rabbis and the Church fathers: demons are the souls of the offspring of angels who cohabited with humans. According to this story, a group of angels descended from heaven and mated with human women, producing as offspring a race of wicked →giants who conquered and defiled the earth with violence and bloodshed. To destroy them, God caused the Flood. The spirits of the drowned giants, neither angelic nor human, were trapped in the regions of the air which they haunt as demons, seeking host bodies to inhabit (cf. “the power of the air” Eph 2:2; and Eusebius, Praep. Ev. 4.5.142: [Greek theologians] assigned “the atmosphere to demons”). According to Justin Martyr, “the angels … were captivated by love of women and engendered children who are called demons” (2 Apol. 5; cf. Gen 6:1–4; 1 Enoch 6–21; Jubilee. 4:22; 5:1ff.; Jude 6).

THEY CAN ENTER HUMANS
Regardless of ancient conjectures, the most accurate definition of the word ‘demon' in the New Testament is a spirit without a body that is able to enter humans, animals and objects and affect behavior or action. They are able to do this in large numbers as was the case of the Gerasene demoniac. The main effect of demons on the host in the Synoptic writers was to cause physical and mental suffering and anti-social behavior. Rather than someone being “possessed of a demon” as we see in the King James Version, the most accurate translation is “to be demonized” or “to have a demon.”

The word is wrongly translated “devil” or “devils” in the KJV. Who many call the devil is really Satan, and the demons, who are under his influence, are also called unclean or evil spirits.

A close study of the teachings of Jesus and His encounters with demons reveal they have distinct personalities.

They can speak (Mark 1:24; 5:7-12)
They have a will (Matt. 12:44; Mark 5:11-12)
They have knowledge (Mark 1:23-24: Act 19:15)
They have emotions (James 2:19)

Demons are unquestionably real and able to invade or influence the minds and bodies of humans including Christians.

Source:  http://keydeliverance.org/WHAT%20ARE%20DEMONS/index_whataredemons.html