Terminology

TLD - top level domain; the domain ending (.com, .xyz, .io, etc)

Domain - the main address for a site (domain.com)

Subdomain - a preface for a domain (subdomain.domain.com)

Registry - a smart contract containing all issued domains

Registrar - a smart contract controlling a domain you can register subdomains in

Root Registrar - a registrar for a TLD

Registrant - the owner of a registration; the user buying a domain

Resolver - a smart contract storing the data for a given domain