The following is the purpose of a Registry;
All registered domain names are listed in a database controlled by a registry, this top-level authority provides rules and regulations for all domain names registered through a licensed registrar. They also hold a record of the registrant details for each domain name, including the contact details of the named domain owner, the administrator of that domain, and a technical contact which will usually be the registrar. The registry provides resources for dispute resolution, be that between registrant parties, or between the registrant and the registrar.
The following is the purpose of a Registrar;
Our company is an example of a registrar. We provide a domain name registration service, under the guidance of a top-level registry (detailed above). We are the first point of call for any queries regarding domains bought through ourselves. We relay all information regarding the ownership of domains to the registry, and are accredited in a way that we are authorised to add, modify and delete this information.
The following is the purpose of the Registrant;
If you're registering a domain name for your website as an individual, once you have paid the appropriate registration fees, you become the official registrant of that domain. Whilst the domain is active and not expired, this gives you control of how the domain is used, and which website it is assigned it. You're in control of the DNS for that domain, and have full control over what content it points to.