From BiblePortal Wikipedia

Bridgeway Bible Dictionary [1]

Both Old and New Testaments speak of the reality of the spirit world. Within this unseen world are spirit beings beyond number. They are commonly called angels, some of whom willingly serve God, though others have rebelled against him ( Judges 1:6; see Angels ). These rebellious, or fallen, angels are variously known as demons, evil spirits, spiritual hosts of wickedness, principalities, powers, rulers, authorities, evil spiritual forces, cosmic powers of evil, and angels of the devil. Their leader is Satan ( Luke 10:17-18;  Ephesians 6:12;  Colossians 2:15;  James 2:19;  Revelation 12:9; see Satan ). In relation to the ministry of Jesus, demons are usually called unclean spirits ( Matthew 10:1;  Matthew 10:8;  Mark 6:7;  Mark 6:13; see Unclean Spirits ).

Demons oppose God and are the spiritual force behind heathen religions and false gods ( Deuteronomy 32:17;  Psalms 106:37;  1 Corinthians 10:19-20;  Revelation 9:20). Those who follow these religions usually fear demons, and often use magic and sorcery to resist demonic power. But God forbids magic and sorcery, for these things themselves involve cooperating with supernatural evil powers ( Leviticus 19:26;  Leviticus 20:6;  Galatians 5:20;  Revelation 9:20-21;  Revelation 21:8; see Magic ). The only dealing Jesus would have with demons was one of total condemnation. He brought God’s judgment upon them and freed demonized people from their power ( Mark 1:27;  Luke 8:2;  Luke 13:32;  Acts 10:38).

Jesus’ power over demons was a sign that the kingdom of God had come ( Matthew 12:28; see Kingdom Of God ). He gave his followers the power to cast out demons ( Matthew 10:1;  Matthew 10:8;  Luke 10:17-18;  Acts 16:16-18), and the ability to see whether prophets in the church were speaking through the influence of demons or the influence of God’s Spirit ( 1 Corinthians 12:3;  1 Corinthians 12:10;  1 Timothy 4:1;  1 John 4:1-3).

Not all demonic activity is spectacular. Demons are busy in every sphere of human life and readily work through the normal activities of human nature and human society ( 2 Corinthians 11:14-15;  Ephesians 2:2-3;  Ephesians 6:12;  James 3:15). Christians should be aware of their evil purposes, but should not fear them. Christ has conquered them by his death and resurrection, and Christians can claim that conquest for themselves ( Colossians 2:8-10;  Colossians 2:15; cf.  Romans 8:38-39;  Ephesians 1:18-21). The final demonstration of the conquest of demons will be at the last judgment, when God’s punishment will remove them from human society for ever and send them to a place of fitting punishment ( Matthew 8:29;  Matthew 25:41;  Judges 1:6).

People's Dictionary of the Bible [2]

Devils, Demons . Although there is only one being known as the devil, the English version of the Scriptures often uses the words devil and devils, where it should read demon and demons. The words in the original Greek are different from the word used when the devil is referred to. Frequent accounts are given of persons possessed by demons, and of our Lord casting out demons. They are evil spirits plainly distinguished from the persons whom they possess, sometimes a number of them entering into a person.  Luke 8:1;  Luke 8:30. They can go out of persons and can enter into swine.  Matthew 8:31;  Mark 5:11-13. They have a separate consciousness; they know the Lord Jesus Christ,  Luke 4:41;  Luke 8:28, and are addressed by Christ as personal beings,  Mark 1:24;  Mark 5:9; and they are looking forward to the day of judgment, to them the day of torment  Matthew 8:29;  James 2:19. As frequent accounts are given, in the Old Testament and in the New, of the devil and of demons entering into persons, there is no reason to doubt that they do so now.

Smith's Bible Dictionary [3]

Demons. In the Gospels, generally, in  James 2:19, and in  Revelation 16:14, the demons are spoken of as Spiritual Beings, At Enmity With God, And Having Power To Afflict Man, Not Only With Disease, But, As Is Marked By The Frequent Epithet "Un-Clean," With Spiritual Pollution Also.

They "believe" the power of God, "and tremble;"  James 2:19, they recognized the Lord as the Son of God;  Matthew 8:29;  Luke 4:41, and they acknowledged the power of his name, used in exorcism, in the place of the name of Jehovah , by his appointed messengers;  Acts 19:15, and they looked forward, in terror, to the judgment to come.  Matthew 8:29. The description is precisely that of a nature akin to the angelic in knowledge and powers, but with the emphatic addition of the idea of positive and active wickedness.