What is the difference between a Registry, a Registrar and a Registrant?

Registry

The operator of a top-level domain is called a Registry.

For example, the registry for .COM and .NET domains is Verisign Inc, for .EU domains it is EURid, and for .RO domains it is ICI - RoTLD.

Agent/Partner or Registrar

The operator of a top-level domain does not normally work directly with end customers, but delegates these operations to partner companies (agents), known in English as registrars. Partners must meet certain financial and technical requirements in order to register domains with a given operator.

Domain holder or Registrant

The end customer who registers a domain through an agent/partner is called, in English, the Registrant. They hold the right of use over a domain for as long as they pay the annual fees on time.