Most people have some sort of compulsion. Some people however, are bordering on mania
A compulsion might be something
that you feel forced
to do, or which makes you anxious
if you don't.
Placing objects in an exact order.
Making sure all the books in the bookcase are aligned correctly.
Sorting all your CDs/LPs often*
Defragmenting your HD often*
* More than twice a week.
People suffering from extreme cases of compulsive behavior might respond violently if they can not act on their compulsion.
I suffer from a mild cpmpulsion myself; if my CDs aren't stacked correctly I feel forecd to correct it, or if my CDs isn't inside their cover I put them there promtly. This is an example of a normal everyday compulsion, and does only apply to my CDs.