A "Registrar" (or "Domain Name Registrar") is an organisation like Register365.com, Network Solutions, Enom or Godaddy that has control over the granting of domains within certain TLDs (top level domains, like the generic .com/.org/.net or country-specific .ca/.us/.mx etc.).Registry:
The 'Registry' is the back end that registrar's have shared access to. Each registrar writes new names to a central registry database, from which the authoritative root (essentially, a table of all domain names on the Internet) is built.Registrant:
A 'Registrant' is the person or company who purchases a domain name. For example, Jessica Smith (registrant) registers the name jessicasmith.com through Register365.com (registrar) who in turn writes the name to the central database (Registry).
Many of the companies selling domains online are Resellers of the main Registrars.