Hello Elisabeth,
Well, in regards to who demons are, we know that demons are just another designation for fallen angels. Consider the following:
"3. Then another sign appeared in heaven: a huge red dragon with seven heads, ten horns, and seven royal crowns on his heads. 4. His tail swept a third of the stars from the sky, tossing them to the earth."
Regarding the above, we know that the "Stars" are symbolic here for fallen angels, as they are identified as such in the following scripture. In fact Rev.12:1-6 is a summary, where Rev.12:7-17 is a detailed account:
"7. Then a war broke out in heaven: Michael and his angels fought against the dragon, and the dragon and his angels fought back. 8. But the dragon was not strong enough, and no longer was any place found in heaven for him and his angels. 9. And the great dragon was hurled down—the ancient serpent called the devil and Satan, the deceiver of the whole world. He was hurled to the earth, and his angels with him.
The "dragon" in verse 3 is identified in verse 9 as "the ancient serpent called the devil and Satan" and the "stars" mentioned in verse 4 that the dragon's tail which tosses them to the earth are identified in verses 7-9 as "Satan's angels." For a little more information regarding demons being another designation for fallen angels, consider the following:
"They had tails with stingers like scorpions, which had the power to injure people for five months.
11They were ruled by a king, the angel of the abyss. His name in Hebrew is Abaddon, and in Greek it is Apollyon." (Both mean Destroyer)
The above is an excerpt from the 5th trumpet, where that "star/angel" opens the Abyss and lets out those demonic beings described as representing locusts, like horses prepared for battle, teeth like lions teeth, hair like women's hair, tails and stings like that of a scorpion. These are also demonic beings who are let out of the Abyss at the sounding of the 5th trumpet and their king is said to be the "angel" of the Abyss, which is non other than the beast who will come up out of the Abyss.
Other designations for fallen angels/demons are unclean spirits and familiar spirits. There are other fallen angels spoken of by Peter and Jude, who have been restricted in a place often misinterpreted as "Hell" called "Tartaroo/Tartarus." I personally believe that, these are the angels who are briefly mentioned in Gen.6:1, referred to as "the son's of God," also called "The watchers" by Enoch, who took wives from the progeny of mankind and so defiled themselves, they being 200 in number were restricted under the earth until the great white throne judgment:
"For if God did not spare the angels when they sinned, but cast them into Tartaroo, delivering them in chains to be held in gloomy darkness until their judgment;" (2 Pet.24:4)
"And the angels who did not stay within their own domain, but abandoned their proper dwelling, He keeps under darkness, in eternal chains for judgment on that great day. " (Jude 1:6)
"They sailed to the region of the Gerasenes,[SUP]
b[/SUP] which is across the lake from Galilee.
27When Jesus stepped ashore, he was met by a demon-possessed man from the town. For a long time this man had not worn clothes or lived in a house, but had lived in the tombs.
28When he saw Jesus, he cried out and fell at his feet, shouting at the top of his voice, “What do you want with me, Jesus, Son of the Most High God? I beg you, don’t torture me!”
29For Jesus had commanded the impure spirit to come out of the man. Many times it had seized him, and though he was chained hand and foot and kept under guard, he had broken his chains and had been driven by the demon into solitary places.
30Jesus asked him, “What is your name?”
“Legion,” he replied, because many demons had gone into him.
31And they begged Jesus repeatedly not to order them to go into the Abyss.
"When an
unclean spirit comes out of a man, it passes through arid places seeking rest and does not find it. Then it says, ‘I will return to the house I left.’
25. On its return, it finds the house swept clean and put in order.
26. Then it goes and brings seven other spirits more evil than itself, and they go in and dwell there. And the final plight of that man is worse than the first.”