Obsessive-compulsive disorder (OCD) is characterised by intrusive thoughts or images (obsessions), which increase anxiety, and by repetitive or ritualistic actions (compulsions), which decrease anxiety. The most recent revision of the diagnostic criteria for obsessive-compulsive disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) emphasises that compulsions can be observable behaviors or mental rituals
The most frequent symptoms in obsessive-compulsive disorder are contamination concerns with consequent washing, or concerns about harm to self or others with consequent checking. Additional research indicates that other concerns may include problems with symmetry, arranging rituals, and hoarding. Many obsessions and compulsions have been identified, including sexual, religious, musical and somatic symptoms. Obsessive-compulsive disorder symptoms have varied little by time or place. Similar symptoms are seen in other cultures. Although a person's predominant symptoms can change with time, typically symptoms do not change by much.
One of the best books on obsessive-compulsive disorder is Brain Lock, by Jeffrey Schwartz, M.D. His explanation of cognitive-behavioral group therapy for OCD is clear and compelling. You can look at this book and purchase it through The Anxiety Network's Bookstore.