Obsessive-compulsive disorder (OCD) is an illness that affects thoughts and actions and is believed to be rooted in a biochemical imbalance of the brain. OCD is classified as an anxiety disorder in the Diagnostic and Statistical Manual (DSM-IV) published by the American Psychiatric Association.

This puzzling illness is characterized by recurrent and disturbing thoughts (called obsessions) and/or repetitive, ritualized behaviours that the person feels driven to perform (called compulsions). Obsessions can also take the form of intrusive images or unwanted impulses. The majority of patients have both obsessions and compulsions, but a minority (about 20 percent) have obsessions alone or compulsions alone (about 10 percent). The person with OCD usually tries to actively dismiss the obsessions or neutralize them by engaging in compulsions or avoiding situations that trigger them. In most cases, compulsions serve to alleviate anxiety. However, it is not uncommon for the compulsions themselves to engender anxiety, especially when they become very demanding.

A hallmark of OCD is that the person recognizes that her thoughts or behaviours are senseless or excessive. However, the drive can be so powerful that the person caves in to the compulsion even though she knows it makes no sense.

Source: http://www.wikipedia.com