What are "thick" and "thin" registry models?

The Afilias Registry is operated as a "thick" registry. A thick registry is one in which all of the information associated with registered entities, including both technical information (information needed to produce zone files) and social information (information needed to implement operational, business, or legal practices), is stored within a central registry repository. The registry-registrar protocol supported by Afilias is the Extensible Provisioning Protocol.

In contrast, registries that support a "thin" model only store the domain and name server information within the registry; contact information is stored at each registrar. As a result, each registrar must operate its own WHOIS server in order to provide access to this information.