The Diagnostic and Statistical Manual defines obsessions and compulsions as follows;
Obsessions are persistent ideas, thoughts, impulses, or images that are experienced as intrusive and inappropriate and that cause marked anxiety or distress. Compulsions are repetitive behaviors (e.g., hand washing, ordering, checking) or mental acts (e.g., praying, counting, repeating words silently) the goal of which is to prevent or reduce anxiety or distress, not to provide pleasure or gratification.
There are several generally accepted subdivisions of OC's. Amongst them are Washers, Checkers, Cleaners, Hoarders, Repeaters, Orderers, and Pure Obsessives.
Washers are those OC's that generally have a fear of germs, dirt, or contamination from substances like bodily fluids, dirt, dust, bacteria, viruses, excretions, and the like. Washers that are compulsive can spend hours washing themselves, or parts of their body, to the exclusion of all else, trying to rid themselves of "contamination". They may also avoid contact with things to avoid being "contaminated".
Cleaners are those OC's that feel that other things are contaminated or dirty, and spend much time cleaning their surroundings. For instance, a cleaner might spend hours dusting their home, and then go back and start again as soon as they have finished, because dust has settled in the meantime.

Checkers have a problem remembering or being sure that they have or have not done something, and therefore go back to check whether they have or not. For instance, a woman may get up 10 to 15 times a night to make sure all windows and doors are still locked, although they were every other time.

Hoarders collect things...almost anything. They usually cannot even stand to throw away garbage, and often will let it just sit around them. An inability to get rid of things is the significant symptom of this class of OC.
Repeaters are OC's that feel compelled to do things a "right" number of times. This may serve to protect them from some imagined danger, or prevent possible harm to themselves. Repeaters generally fear that if they do not do things the "right" number of times, something bad will happen, although some may just have to do things "just right" for no apparent reason.
Orderers have to have things organized absolutely "the right way". An orderer might be reluctant to let anyone touch their possessions, lest they be misarranged. Orderers might spend hours just aligning a piece of paper on a desktop, or straightening a bookshelf.
The last type is the Pure Obsessive, which is also the most difficult OC to treat. These OC's suffer from obsessive thoughts of a disturbing nature, generally. An example might be a person who constantly obsesses over whether they will hurt their child, even though they know they wouldn't...they can't stop worrying that they might.
Ian appears as a very intelligent young man to me. He just can't seem to keep on task. He doesn't appear to be lazy in the least. He tells me that his reason for his inability to stay focused is because his mind keeps telling him he has to do all kinds of crazy things such as; how he has to count how many breaths he takes, or how many steps he has taken lately. Also that he has to hold his breath every time he passes a locker when the number on the front of it is divisible by five. He complains that he is terrified by doorknobs, water faucets, light switches, or air vents because of the germs they may possess. If he touches a doorknob, water faucet, or light switch, he immediately has to wash his hands seven times; he claims seven is his magic number. He says he feels better everytime he washes his hands, but he is not totally satisfied because there are so many germs out there. A big fear he has is the one that he may catch AIDS just by making contact with one of the above objects. He claims to realize that no one else seems to worry about the things he does and he knows that sometimes the things he does do not make sense, but he just can't shake these obsessions.

About 2.3% of the U.S. population (3.3 million Americans) has OCD in a given year. OCD affects men and women equally. OCD typically begins during adolescence or early childhood; at least one-third of these cases of adult OCD began in childhood.

Treatments for OCD have been developed through research supported by the NIMH and other research institutions. These treatments, which combine medications and behavioral therapy, are often effective. Several medications have been proven effective in helping people with OCD such as, clomipramine, fluoxetine, fluvoxamine, and paroxetine. A number of other medications are currently being studied.

A type of behavioral therapy known as "exposure and response prevention" is very useful for treating OCD.

The treatment I found to be most effective, after reading all the studies I read was the behavioral therapy. In this approach, a person is deliberately and voluntarily exposed to whatever triggers the obsessive thoughts and then, is taught techniques to avoid performing, the compulsive tasks and to deal with the obsessions. Studies of behavioral therapy for OCD have found it to be very effective treatment to those who complete it. For the treatment to be successful, it is important that the therapist be fully trained to provide this specific form of therapy. It is also helpful that the patient be highly motivated, be positive and have a determined attitude.

? Diagnostic and Statistical Manual (300.3 Obsessive-Compulsive Disorder)