A registrar is the company or organisation that you choose to register your domain name through. This may be an Internet Service Provider (ISP), a domain name reseller or a company that specialises in registering domain names and protecting intellectual property and brand rights.
Registrars are also known as 'Registration Agents' or 'Tag Holders'.
A registrar is the agent through which people register domain names – it does not mean that they are an agent of Nominet. Your registrar may also be a member of Nominet but they will act on your behalf rather than Nominet’s behalf.
Registrars have contractual obligations to both registrants and Nominet and you will enter into an agreement with both your registrar and Nominet when you register a .uk domain name.