Obsessions are persistent ,intrusive, nonsensical thoughts that you can’t shake out of your head. For example, you may worry that the dirt on your hands will contaminate your children and give them leukemia , even though you know rationally that this is not realistic.
A compulsion is ritualistic act that you feel compelled to repeat over and over in order to ward off the danger . Thus , the obsession is the repetition , frightening, thought and the compulsion is the repetition action that results from that thought .
According to the diagnostic and statistical manual , the most common kinds of obsessions concern violence, contamination, or doubt.