Fallen angels are what the name implies.
Demons came from the Genesis giants, the Nephilim, who were the offspring of fallen angels and human women. When they were killed, since they were half human kind and half angel kind, they did not immediately go to the dark dungeons with everlasting chains Peter describes. They were left to roam the earth and have Satan as their leader.
Unclean spirits are usually thought of as demons.
Something to consider, God locked up all the angels who sinned who went after strange flesh, those women, so they are not free to roam, they are there until judgement day!
So what we are left with, some fallen angels, like Satan, did not do that particular evil thing, and are still free to roam.
The question all should ask themselves what evil thing did one particular group of fallen angels do that the other fallen group did not do to make God put the one group into everlasting chains. Genesis and Jude makes the case for them creating the Nephilim which turned into the demons when they were killed.
Another thing, these demons seek out a body to possess, its called demonic possession because they miss their body it having been destroyed, (the fallen angels never had their own body being complete without a body of flesh) and they are unclean evil disembodied spirits trying to find a new home. Because like Jesus said, when they are out of the body, they travel through waste places and seek rest but cant find any. His goal is to get back into another body, or the same one again if possible, because that was his home for rest. Perhaps when disposed spirits are forced out, they are in distress, they are definitely uncomfortable.
Matthew 12
43 “When an unclean spirit goes out of a man, he goes through dry places, seeking rest, and finds none. 44 Then he says, ‘I will return to my house from which I came.’ And when he comes, he finds it empty, swept, and put in order. 45 Then he goes and takes with him seven other spirits more wicked than himself, and they enter and dwell there; and the last state of that man is worse than the first. So shall it also be with this wicked generation.”
Jude,
6 And the angels who did not keep their proper domain, but left their own abode, (they fell), He has reserved in everlasting chains under darkness for the judgment of the great day; 7 as Sodom and Gomorrah, and the cities around them in a similar manner to these, (the fallen angels) having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the vengeance of eternal fire.