Yadis is a communications protocol for discovery of services such as OpenID, OAuth, and XDI connected to a Yadis ID. While intended to discover digital identity services, Yadis is not restricted to those. Other services can easily be included.
A Yadis ID can either be a traditional URL or a newer XRI i-name, where the i-name must resolve to a URL. The so called Yadis URL either equals the Yadis ID (if this is a URL) or the resolved URL of the XRI i-name.
Furthermore, Yadis specifies how to use the Yadis URL to retrieve a service descriptor called Yadis Resource Descriptor. This descriptor follows the XRDS format and connects several services, like authentication or authorization to the Yadis URL. Each service description can have further parameters.
Read more about Yadis: Modular Architecture, Discovery of The Yadis Resource Descriptor, XRDS Documents, LID, OpenID and The Developer Community