The different actors within the domain name supply chain all have similar names.
The registry is the operator of the Top Level Domain, and is responsible for maintaining the database of all domain name registrations and their associated IP addresses (equivalent to a wholesaler). The registry is the authoritative entity for that Top Level domain, and is included in the “root” directory, the Internet Assigned Numbers Authority (IANA) database. Most of the larger registries included in this study do not have direct interaction with end users at the domain name registration phase.
Registrars sell domain name registrations to end users (retailer). They typically also provide a number of other services, and many offer a range of different TLDs to their customers. Registrars are usually accredited or otherwise authorised by a registry to sell individual TLDs.
Registrants are the people or organisations who register domain names for their own use (customer).