Obsessive Compulsive Disorder (OCD)
You have been laying in bed for almost an hour trying not to go outside and turn your car on so that you can change the radio station so that when you start the car in the morning, the right station will be on. You have an overwhelming feeling that something bad will happen if you don't do it. You know that your thoughts are not logical, but you just can't shake the feeling. The feeling is overpowering. Finally, you go change the radio station and hope that no one wakes up, so you won't have to explain.
Obsessive Compulsive Disorder (OCD) is characterised by intrusive and obsessive thoughts like the ones in the example above. These thoughts cause anxiety that mounts until a behaviour is performed in an effort to reduce the anxiety. Often these behaviours take the form of ritualistic and/or repetitive behaviours that are carried out in order to prevent or produce a particular situation. These rituals can become so rigid and repetitive that they become distressing and significantly interfere with a person's quality of life. Often times, the individual understands that their thoughts and actions are not logical. Regardless, the individual feels the related anxiety as if the thoughts are accurate and potential outcomes are real.