Demons are real, but the biblical description of them and their activities is a far cry from the imaginations of authors and Hollywood writers.
Demons are fallen angels who rebelled against God with Satan. They belong to the kingdom of Satan, and thus are at cross-purposes with God. While they are stronger than men, they are in no way equal to God in power or glory. Demons in the New Testament recognize Jesus as the holy one of God, and know their ultimate fate is eternal punishment.
They are purely spiritual beings and there is no record of demons taking on a physical form of their own. There are several instances of demons possessing human beings, though again, the resemblance between what the Bible describes and what our pop-culture imagines is minimal at best.