Obsessions are persistent ideas, thoughts, impulses, or images that are experienced as intrusive and inappropriate (unless you are delusional). Obsessions usually cause anxiety or distress. Common obsessions are: contamination (getting
germs), doubts (did I turn off the stove), need to have things in a particular order (reorganizing the kitchen cabinets alphabetically), aggressive impulses (stalking), and sexual images.
Source: DSM IV-TR
To read a mystery about Obsessions, check out Object of Obsession