The domain name registrant is the organization or individual registering the domain name. When you submit your application for a domain name, you become the domain name registrant.
A registrar is the company that takes your registration request and reserves your domain for you at the main registry.
During the domain name registration process, you will be asked to provide the registrar with contact and technical information. The registrar will then submit the information to the central directory, also known as the registry. The registry enters your information into their database so that other computers on the Internet can look-up the necessary information to find your website.