Domain Name Registration
Domain Names are normally registered through a Domain Name Registrar. This company manages the domain name for the registrant, including contact details, registration/renewal payments, and providing the means to change the Name Server attached to the domain name.
The Domain Name Registration only secure the relevant term as a domain name eg example.com or example.co.uk - it needs to be attached to additional services to be used for a web site or email and the like.
The Domain Name Registrar may well provide Name Server services of their own, so that a domain name can be pointed to the relevant company that provides the space on the internet where the content resides. This is called hosting, and many Registrars provide this too as an additional costed service.
The Domain Name Registrar deals directly with the underlying Registry that is responsible for the Top-Level Domain Name in question eg Verisign for .com or Nominet for .uk.
Domain names are usually registered for distinct periods of time, requiring a renewal payment to be made at the end of that period to maintain the registration.
There are 2 types of Domain Name Registry - Thick Registries and Thin Registries.
Thick Registries are where the Registry maintains the contact details for the Registrants, and the Registrars make changes to the Registry held information, .uk is a thick registry.
Thin Registries are where the Registrars maintain the contact details themselves - and the Registry simply points people to the relevant Registrar in question when a party wishes to see who owns a particular domain name. .com is a thin registry.
All that is needed for the technical functioning of a domain name is to know which Name Server has authority - this information is held by the Registry in both thick and thin registries.
Most registries operate a WHOIS service, which is a means to determine who registered a particular domain name. In the case of a thin registry, each Registrar must operate a WHOIS service of their own to provide access to that information, whereas thick registries have that information available at the Registry level.
Not all companies offering domain name registration services have direct relationships with the Registries - many are resellers for other Registrars.