The Domain Name System (DNS) is an Internet system where alphanumeric strings (domain names) can be used to locate Internet resources. The Domain Name System (DNS) is comprised of a root zone (name-less), top level domains, second level domain and third level domains etc. Examples of top level domains are: com, org, and net. ICANN assigns the organisations who manage the top level domains and who can register namespace (domain name) in these top level domain. The companies who can register namespace in top level domains are called: Domain Name Registrars or simple Registrars.
The organisation who manage each top level domain usually charge a set fee for a namespace in their domain. Registrants cannot buy a namespace directly from the manager of the top level domain. Instead registrars - which are accredited by ICANN - purchase the namespace from the top level domain manager on behalf of the end-user. The registrar also manages the DNS record of the domain name for the registrant. Top level domain operators and registrars usually make a profit during this transaction; how much, is dependent upon the organisation. The rules by which the registrar operates are set by ICANN. ICANN has ultimate authority over domain name disputes; if an end-user (named a registrant) feels an accredited registrar has breached ICANN's domain name rules they can submit a complaint to ICANN.
The registrar system was officially launched by ICANN on the 30th of November, 1999. One of the first registrar's accredited by ICANN was Godaddy. There are currently (2014) over nine hundred accredited registrars. A list of accredited registrars was published at the InterNIC website; although this list may be out of date. The InterNIC website heavily redirects to the ICANN website; and may be defunct (as of 2014). Registrars pay a fee to ICANN to become a registrar; an individual/company can apply to become a registrar by visiting the ICANN website and applying through their instructions. Some accredited UK registrars include: 123-Reg Limited, Astutium Limited, BB-Online UK Limited, Demys Limited, Discount Domains Ltd., Domainbox Limited, Domainmonster Limited, Domains.coop Limited, Easyspace Limited, LCN.COM Ltd., Lexsynergy Limited, LiquidNet Ltd., NameCamp Limited, NetEarth One Inc. d/b/a NetEarth, Netistrar Limited, Nominet Registrar Services Limited, Paragon Internet Group Ltd t/a Paragon Names, TLD Registrar Solutions Ltd, and UK-2 Limited.
The vast majority of registrars are based in the United States of America; which is not surprising as that was the country the Internet was developed in. The list of registrars is always changing. When a registrant registers a domain name on the Internet, the company who is doing so on behalf of the registrant may not be an accredited registrar of ICANN. Instead, they may using the infrastructure of an accredited registrar and are in reality a third party registrar. These third party registrars (resellers) usually do not provide full access to registrants to change DNS records via the WHOIS protocol. Accredited UK registrars like 123-Reg Limited provide full DNS control to registrants.