The Mayo Clinic explains that "OCD" stands for "obsessive-compulsive disorder." It is an anxiety disorder in which a person has unreasonable thoughts and fears, called obsessions, that cause him to perform repetitive behaviors, called compulsions, according to the Mayo Clinic.
According to the National Institute of Mental Health, a person who is obsessed with dirt or germs may reel compelled to wash his hands repeatedly, while a person obsessed with intruders may feel compelled to check the locks on his door repeatedly,
Per Psych Central, most people who suffer from this condition have both obsessions and compulsions, but some only have one or the other. People with OCD often realize that their thoughts are unreasonable, but they are powerless to control them.