Demons are often referred to as "fallen angels," or, "Satan's angels."
Satan was the very highest angel, but made the mistake of rebelling against God and was cast out of heaven, (Is. 14:12-15).
The evil angels who followed Satan were also cast out. They are the demons.
These evil beings represent the very depths of evil, including hatred, bitterness and perversions. Lust, homosexuality, drunkenness, gluttony and witchcraft. Though often an expression of sinful flesh, they can also express their presence in the lives of people, through perversions such as sadomasochism and pedophilia.
Their purpose is to torment and harass people, leading them away from God and His truth. The bible gives the following example:
"...This man lived in the tombs, and no one could bind him any more, not even with a chain. For he had often been chained hand and foot, but he tore the chains apart and broke the irons on his feet. No one was strong enough to subdue him. Night and day among the tombs and in the hills he would cry out and cut himself with stones" (Mark 5:2-5).
Just as there are archangels in God's kingdom with higher powers, there are principalities and powers" that rule over Satan's angels. There is constant conflict, and battles between Satan's principalities and powers and God's loyal angels.
We are told that our prayers offer up a restraint against their evil activity by summoning the intervening presence of God. (Dan 10:12-13).
Possession is not the same as being influenced, or attacked by them. To be "possessed," means to be indwelled by, and under their complete control. However, they regularly attack and attempt to trouble and influence mans' behavior, even God's people.
Christians must guard not only against the sin of the flesh, but also their influence. These evil spirits can influence and intensify the works of the flesh. This must be recognized and stood against before one can die to the flesh and walk in the Spirit.
Christians need not fear Satan or his fallen angels. We are told "...the one who is in you is greater than the one who is in the world." (1 John 4:4)