Nominet are the organisation who regulate all the Internet Domains in the UK namespace. Here is a link to their site http://www.nominet.org.uk/
If you have any questions about UK domains or the way they are administered check out the Nominet web site- the answer is there!.
The UK Namespace refers to all the Internet Domain named ending .UK (dot UK). Domain name are often associated with a web site name and often with one or more email addresses. But there are other more specialist uses for Domain names too.
DNS is short for Domain Name System. The DNS is part of the internet generally hidden from most users but is crucial for the smooth operation of the whole internet. The DNS connects a Domain Name to the actual servers which hosts the internet service (for example the web site). A single Domain name may need to be connected to more than one server. For example a web site may be hosted on a different server to the email.
I take the view that a good registrar should deal with the technical DNS issues so the web site owner doesnt have to. In fact many web site owners don't not have the slightest idea that DNS even exists!
The IP number uniquely identifies a particular server on the Internet. In an analogy with the telephone system the IP number is like the Telephone number and the DNS is like the telephone directory.
A nameserver is a particular kind of server. It holds all the DNS details about your domain and and the IP number of the server hosting your web site or other services such as where to send any email addressed to your domain.
When a registrar registers your domain name he has to inform the Authority for the Namespace (Nominet for UK domains) the location of a master nameserver. This is called the Authoritative nameserver. At intervals other nameservers around the internet copy this information so that eventually all the nameservers around the internat have the same information about your Domain. It often takes around 24 hours for the DNS information to propagate around the world and this is usually the reason for the delay if you change the location of your web site.
All registrars contact the Namespace Authority (Nominet for .UK domains) and register your name against an unused domain name. This becomes the Domain Name used for your Web site. Fees are payable annually for the continued use of the Domain name. The registrar often collects the fee from you and passes it to the Namespace Authority on your behalf. If the fee is not payed the Domain is eventually passed back into a pool and becomes available for use by anyone else requesting it.
A registrar will notify you whan such fees are due. A good registrar is also responsible for maintaining at least two Authoritative nameservers (more than one is needed for redundancy in case of server failure- I use 3 Nameservers).
Keeping the Namespace Authority (Nominet) updated on their location and on keeping the master DNS records updated so they can be copied around the world to other nameservers.