Let me try and give a brief answer to your question. You will notice that I use the King James "devils" instead of the transliteration "demon." I am not criticizing, just staying with the biblical terminology. Also, I admit that the distinctions that I am getting ready to give you are not agreed upon. They are my best conclusions according to the Bible. The spiritual beings that are in opposition to God include:
- The Devil
The fallen angels that actively serve the devil at this time. These are ye to be cast out (Revelation 12:9).
The fallen angels that are presently in chains (2Peter 2:4; Jude 1:6). These are a special group of fallen angels who are given to greater judgment. They "kept not their first estate" (2Peter 2:4). That is, they did not keep their home in heaven. They seem to be connected with the destruction of the world in Noah's day (2Peter 2:4-5). I believe this identifies them as the sons of God who came down and married with the daughters of men in Genesis 6:1-4. Because of their perversion in seeking the daughters of men, they received a greater and more immediate punishment for their sins.
The devils. The devils are never called fallen angels. They have attributes that are never attributed to angels. The most significant attribute is their desire to inhabit a body (Matthew 8:28-31). Though angels can take on a body of flesh, they have no need to do so because they are created as spirits (Hebrews 1:14) and are complete in that form. The devils are spirits too, being called evil spirits (Luke 7:21). But they are not complete in spirit form. They are continually seeking out a body to indwell. Matthew 12:43-45 tells of the search of an unclean spirit for a body to inhabit. This type of language is never used with angels.
Of course, if devils are not fallen angels, the question arises as to who they are. The logical answer would be that they were beings that once had bodies since they now seek to inhabit one. My guess (and it is a guess) is that they are spirits of earlier inhabitants of an earlier earth (not men) who rebelled against God and are awaiting judgment--they feared that Christ was come to torment them "before the time" (Matthew 8:29).
That there should be spirit beings in many forms and at many levels should not surprise us. Ephesians 6:12 states, "For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places."