What is OpenID?

OpenID is a decentralized user authentication protocol developed by the OpenID Foundation. OpenID allows users to be authenticated to a Website using a third-party identify management service. In other words, OpenID is a SSO (Single-Sign-On) technology.

As SSO technology, OpenID offers two main benefits:
Eliminating the need for the Website to their own login system, which may not be as good as the third-party service. We can assume that the third-party service provider can do a much better job than individual Website owners.
Allowing the user to log into multiple unrelated websites with a single sign-on identity registered to the third-party service. We can assume that multiple unrelated website owners all uses the same third-party service provider.

For more technical information, see OpenID Website.