Anxiety is what tells our body that danger is near and all of us have some level of anxiety over day to day concerns, at some time or another. In a person with OCD, our body alerts us to danger at times that it shouldn't. OCD tends to run in families, and my understanding of it is that the tendency toward OCD can be inherited, having to do with levels of the brain chemical serotonin, but the actual disorder cannot. It can also be brought on by traumatic experiences. Obsessions are recurrent thoughts that cause anxiety.
Common obsessions are dirt, germs, and orderliness.
Common compulsions are washing, checking, and cleaning. These obsessions and compulsions are performed in an attempt to relieve anxiety, but the anxiety is only temporarily relieved so the compulsive behavior has to be continually repeated. My OCD manifests itself mostly in orderliness, and cleanliness. I am a perfectionist and extremely conscientious, as most people with OCD are, and because of that my OCD bleeds into almost every area of my life. Even though I know it is not reasonable to continually perform these tasks it is still very difficult for me to stop.