Well, I am not 100% sure on this but, I believe that there is 1 central database of all the domains etc. And the domain registrar has access to this database. You buy the domain from them and then that registrat will update its NameServer and then it will go from NameServer to NameServer until it updates the central database and then it does it in the reverse order. That is why it takes 24-48 hours to update DNS. Again i think this is how it works but i am not 100% sure on that.
Hope this helps
Brett