Domain name resolution is a very important building block of the Internet today. It translates domain/host names into IP addresses, so your browser would know how to reach this page (okay, I know it is more complicated than that, but…)
At the same time, a slow or broken DNS system can be very frustrating, even when you are just hosting a service. Little did I realise that how much a typical hosting server depends on domain name services. If you are on a Linux/Unix box, try to remove /etc/resolv.conf from your server for 5 minutes, and see how it can cope.