For many people, the Devil is a personification of evil. The Devil in a dream usually represents something in yourself that you are unhappy about and that you do not want to accept. When you dream about the Devil, you are experiencing a conflict between different aspects of your personality, similar to the ultimate conflict between Good and Evil, but on a lesser scale.
The Devil in a dream can represent basic, animalistic urges that you try to repress when you are awake.
When you have a dream about the Devil, you are being given an opportunity to face and to try to understand things about yourself that you have always feared and kept hidden.
Use this opportunity to learn to accept and deal with these aspects of personality so that you do not lose control of them and become overwhelmed by them.