Domain names are part of the Domain Name System (DNS) - which is a naming system that uses a alphanumeric string to find an Internet resource located at an IP address.
The Domain Name System (DNS) has a hierarchy: the top is the root zone (controlled by IANA) and below that is top level domains that are maintained by organisations who operate a registry (database) (assigned by ICANN). Some examples of top level domains are: org, com, net, and uk. It is possible for end-users to register a namespace within these top level domains; a namespace being a alphanumeric string called a domain name. However, the end-user cannot (usually) register a namespace with the organisation who manages the registry for a top level domain. Instead, accredited registrar's (like tucows) pay a fee to the organisation - who operate the top level domain - on behalf of the registrant and submit data to the registry database of a top level domain.
The company, organisation or individual of a namespace (domain name) in a top level domain is named a registrant. The registrant is viewed as the "owner" of the domain name, but, the registrant requires a registrar to register and manage the domain on their behalf. When a registrant registers a domain name, they have to supply the following contact information: name, address, telephone number, and email address. Registrants will also have to supply a registrant "type"; for uk domain names, the following registrant types are available:
The other data that is supplied to a registry of a top level domain is usually provided by the registrar; such as the authoritative nameservers for the domain name. Some top level domain operator, like Nominet for the uk TLD, allow individuals to opt out of having their postal address published by a WHOIS query. The WHOIS protocol is used to query the registry database of top level domains; registries and registrars usually have to provide a WHOIS service on port 43 to faciliate queries of domain names.
Registrants can transfer management of their domain name to another registrar. Why would a registrant want to do transfer management? some registrar's charge less to renew a domain name, and registrar's vary in the amount of management options they provide for a domain name. Ideally a registrant should have full control over there DNS record. Registrars can charge a fee to registrants to transfer management of their domain name.