Satan and whomever else he directs them to hinder. They are eternal beings and have already been sentenced to Hell and will be placed there after the final judgment. Whereas angels do not need a physical body because they function effectively as spirits, since that is the way God designed them. The Scripture is not clear on this but it is clear that demons exist and are a real factor in our personal spiritual battles. At that time they were not evil.
Demons seem to receive vicarious pleasure from tormenting and generally ruining a person's life.
It seems that many are driven by a particular appetite, and seem to have different interests in the way they torment their victims. They also have names that it appears relate to their individual carnal cravings. At times Jesus asked for them to identify themselves yet at other times he did not allow them to speak. He set free those oppressed by demons. Jesus taught that we were to continue in the example that He showed us.