Obsession is a repetitive and uncontrollable set of emotions, thoughts and beliefs that cause people to continue believing and organizing their behavior accordingly, although they know that they are unrealistic and unfounded.
Compulsion is behavioral rituals that are performed to control obsessional emotions, thoughts and behaviors.
People who try to cope with OCD constantly try to control their environment and try to reduce their anxiety. They engage in performing various rituals (providing symmetry, not touching the door handle, counting numbers, etc.) in order to cope with the negative emotions and thoughts circulating in their mind uncontrollably. These rituals can often reach the level that prevents people’s vital activities.