How are demons different from the devil?

The word "demon" (dai-mon) is found in the New testament and refers to the fallen angels.  New Testament writers also refer to them as "unclean spirits. However, the term "devil" (diabolos), is used almost exclusively in reference to Satan.  The fallen angels are all, without exception, subordinate to the devil or Satan who is their master and overlord.  Chapter 12 of the Book of Revelation tells us that when the devil fell from heaven "his tail swept a third of the stars from the sky and hurled them down to the earth" (v.4), a common allusion to the apostasy of the other angels who rebelled against God and Satan.  The Fathers of the Church understood that these apostate angels came from all ranks of the heavenly heirarchy.

By Fr. Thomas J. Euteneuer "Exorcism and the Church Militant" page 26