Obsession is the wellspring of genius and madness - Michel de Montaigne
Obsessive-Compulsive Disorder (OCD) is much more significant than many people tend to believe, and also much more varied. It is characterized by both obsessions, and compulsions.
Obsessions include intrusive thoughts, images, or ideas that are unwanted. It's important to note that this is VERY different from what many people think of as obsessive. For instance, children are not typically obsessed with Minecraft - because they really enjoy thinking about it. An obsession is something a person doesn't want to think about, but can't seem to stop.
Compulsions include actions that a person performs in response to the obsessions - usually to try to reduce or mitigate the fear or discomfort caused by the obsessions. These are much broader than simply handwashing, and can include very complex or very subtle behaviors.
The most effective treatment for OCD is Exposure Therapy, or ERP. The focus of this therapy is on stopping the compulsive behaviors, and facing irrational fears through the use of exposure exercises.