OCD : Obsessive Compulsive Disorder is basically a serious anxiety related dis-ease of unreasonable thoughts and obsessions that lead to compulsive behavior. Obsessions, in this case, are quite distressing and compel a person to carry out repetitive behaviors and rituals. These include constant reassurance seeking, systematic placement of things and objects, cleanliness, checking of body parts, internal mental counting, repeated religious practices, re-checking doors and locks, spending a lot of time in bath and washing, fearful of falling prey to a shameful act at a public place, harming/humiliating the most loved ones assuming they might insult their religious idols and so on.
All of the above are compulsions and they cripple the individual as they are out of his/her whim. Initially one thinks that , maybe he can control and overcome these. But there comes a time when the situation gets completely out of control and these constantly running unwanted intrusive thoughts start haunting the person to the extent of forcing them to perform their ritual without a choice. Performing them does bring some relief to the distress caused but sadly the relief is only short lived and occurs time and again the individuals obsessive thought gets triggered.
It is based on the theory of perfectionism, meaning that people want to do everything just perfectly. In the background it is operated by the fear underlying.