Registry? Registrar? Registrant? What do these words mean?

Registrar: A "Registrar" (or "Domain Name Registrar") is an organization like Network Solutions 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 backend that registrars 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 serve-you.net, (registrar reseller) who in turn writes the name to the central database (Registry).