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.
Examples:
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.