A registrar is a company that has been accredited by the Internet Corporation for Assigned Names and Numbers (ICANN) and has been verified to sell domain names. The registrar is the middle-man between a registry and a customer. When a domain is requested by a potential customer, the registrar will check with the registry to see if the domain is available. If the domain is available, a request is made to obtain that domain for a customer.
Each Top Level Domain (TLD) such as .co.uk or .com has an organisation in charge for maintaining a database of domain information and is responsible for publishing this information to a WHOIS database available to the public.
A registrant purchases domain names through a registrar for use over a specified term. All registrars offer domains to be renewed by the current owner when the domain is approaching expiration as long as the customer has adhered to the terms and conditions of the registrar.