In Revelation 12:4 it tells us that the dragon (i.e., the devil or Satan) "drew a third of the stars of heaven and threw them to the earth." The "stars of heaven" is a reference to angels. These angels, Jude says "did not keep their proper domain, but left their own habitation" (1:6). Here we are introduced to the subject of fallen angels or demons. All angels, including Satan, are created beings.
Their Origin. When you consider the origin of angels there have been many views other than the one I just gave you from the Bible. The Spirits of Deceased Evil People. This was the view of Philo, Josephus, some early Christian writers, and ancient Greeks (Paul Enns, The Moody Handbook of Theology, p.295). This theory is proved false because evil people are in Hades after death (Lk.16:23). The Spirits of a Pre-Adamic Race. This theory is based on the "gap theory" of an original creation of Genesis 1:1, rebellion and a fall of that originally created race between Genesis 1:1 and 1:2, and the resultant chaos. Genesis 1:3 describes the recreation. The problem with this view is that it depends on a creation of humanity prior to Genesis 1 and 2 and there is no Biblical basis for such a view.
Fallen Angels. This view best supports the Bible. Their Number. A Third (Rev.12:4). Rev.12:4 says it was a "third of the stars of heaven" that Satan "drew." How many is a third? An Innumerable Amount. The Bible says that their are "innumberable company of angels" (Heb.12:22). Rev.5:11 says there are "many angels" or as the NIV says "thousands upon thousands, and ten thousand times ten thousand." A third of the angels would in this case be thousands. The Word "Demon" Appears 33 Times in the New Testament (NIV Quickverse). The Word "Demons" Appears 49 Times in Both the Old and New Testament (NIV Quickverse). Their Activity. They Can Inflict Disease. Luke 13:11 - "And behold, there was a woman who had a spirit of infirmity eighteen years, and was bent over and could in no way raise herself up. Luke 13:16 says that she had been bound by Satan. They War with Believers - Eph.6:10-12 - Finally, be strong in the Lord and in his mighty power. 11 Put on the full armor of God so that you can take your stand against the devil’s schemes. 12 For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. (NIV) They Can Deceive the Church. 1 Tim.4:1-3 - Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; 2 Speaking lies in hypocrisy; having their conscience seared with a hot iron; 3 Forbidding to marry, and commanding to abstain from meats, which God hath created to be received with thanksgiving of them which believe and know the truth. (KJV)
As we have seen Demons are fallen angels. These fallen angels work with Satan in inflicting disease, warring with believers and deceiving the church. As you go throughout your Christian life — remember they were created and they are not omniscient, omnipresent or omnipotent. They can only act as God allows them. 1 John 4:4 says, "You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world."