If you are not an EU national, these guys are going to be very important in your life. If you are, don't be fooled into thinking you need a residency permit. You don't. This is from the IND's document about registration and the EU.
What if I do not register? If you do not register there are no immediate consequences, because according to the EC-treaty, your stay will still be lawful. However, if you do not register this may mean that you will not be able to apply for social security benefits, nor will you be able to apply for student grants or loans.
If you want to know what a 'pronouncement of undesirability' is, click on the website