National Insitute of Mental Health: Obsessive-Compulsive Disorder.
Obsessive-Compulsive Disorder (OCD), is a chronic and long-lasting disorder that causes repeated, upsetting thoughts called obsessions. Someone with OCD may do the same thing over and over again to try to make the thoughts go away. Those repeated actions are called compulsions and can interfere with all aspects of life.
Examples of obsessions are a fear of germs or contamination, a fear of being hurt, or having things in symmetrical and perfect order. Compulsions include washing your hands, counting, checking on things or cleaning.
Studies have shown that people with a first-degree relative who has OCD are at a higher risk for developing OCD themselves.