Obsessive-compulsive disorder (OCD) is a psychiatric disorder most commonly characterized by a subject's obsessive, distressing, intrusive thoughts and related compulsions (tasks or "rituals") which attempt to neutralize the obsessions. Thus it is an anxiety disorder. It is listed by the World Health Organization as one of the top ten most disabling illnesses in terms of lost income and diminished quality of life
Explanation
The phrase "obsessive-compulsive" has worked its way into the wider English lexicon, and is often used in an offhand manner to describe someone who is meticulous or absorbed in a cause (see also "anal-retentive"). Such casual references should not be confused with obsessive-compulsive disorder; see clinomorphism. It is also important to distinguish OCD from other types of anxiety, including the routine tension and stress that appear throughout life. A person who shows signs of infatuation or fixation with a subject/object, or displays traits such as perfectionism, does not necessarily have OCD, a specific and well-defined condition.
To be diagnosed with Obsessive-Compulsive Disorder, one must have either obsessions or compulsions alone, or obsessions and compulsions, according to the DSM-IV-TR diagnostic criteria. The Quick Reference to the diagnostic criteria from DSM-IV-TR (2000) describes these obsessions and compulsions:
Obsessions are defined by:
Recurrent and persistent thoughts, impulses, or images that are experienced at some time during the disturbance, as intrusive and inappropriate and that cause marked anxiety or distress.
The thoughts, impulses, or images are not simply excessive worries about real-life problems.
The person attempts to ignore or suppress such thoughts, impulses, or images, or to neutralize them with some other thought or action.
The person recognizes that the obsessional thoughts, impulses, or images are a product of his or her own mind, and are not based in reality.
The tendency to haggle over small details that the viewer is unable to fix or change in any way. This begins a mental pre-occupation with that which is inevitable.
Compulsions are defined by:
Repetitive behaviors or mental acts that the person feels driven to perform in response to an obsession, or according to rules that must be applied rigidly.
The behaviors or mental acts are aimed at preventing or reducing distress or preventing some dreaded event or situation; however, these behaviors or mental acts either are not connected in a realistic way with what they are designed to neutralize or prevent or are clearly excessive.

More Info & Souce:
http://en.wikipedia.org/wiki/Obsessi...lsive_disorder