If the Registrant is the person who buys the domain name and the Registry is the place from where the domains come from, then the Registrar can be considered the link between them. It is an authorized organization that can register domains on the Registrant's behalf from the domain registry. There are a huge number of choices out there in terms of domain Registrars, and each of them has its specific policies in regards to renewal rates, registration periods, and support.
They each cater registrations for different TLDs and may work together with other registrars to increase their TLD pool. Note that a domain Registrar may also be a Registry for a specific TLD. An example of that is One - they are a domain Registrar and offer registration services for various TLDs. In addition to that, they are the domain registry for the .one domain extension.