An obsession is a thought, an idea, or an image that you cannot get out of your mind.
Common obsessions involve doubts, fears, orderliness, aggression, and sexual images.
A compulsion is a feeling that you have to do something. Compulsions are physical or mental acts that you perform over and over. Common compulsions include counting, washing, arranging and checking things again and again. When these tendencies are exaggerated, it is called an obsessive-compulsive disorder. Obsessive-compulsive disorder affects millions of people.