OpenID is an open standard and decentralized authentication protocol. It allows users to be authenticated by co-operating sites using a third party service, eliminating the need for webmasters to provide their own ad hoc login systems, and allowing users to log in to multiple unrelated websites without having to have a separate identity and password for each.
You can find more information on the openidexplained.com website.
OpenID is decentralized protocol, which means you can login to any OpenID-enabled website using any OpenID provider. Unlike most popupar OAuth providers such as Twitter or Facebook where you have to implement each provider individually.
This service has support for OpenID 2.0, which is the legacy version of the protocol. But it is the most popular version of OpenID implementation among the websites, which offers OpenID authentication.
This service has enabled Yadis discovery protocol. Yadis is a communications protocol for discovery of services such as OpenID, OAuth, and XDI connected to a Yadis ID.