Registrar: A "Registrar" (or "Domain Name Registrar") is an organization like OpenSRS or Register-IT CC 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/.co.za etc.).
Registry: The "Registry" is the backend 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 example.co.za through Register-IT CC(registrar) who in turn writes the name to the central database (Registry).