OpenID is an authentication protocol, wherein OpenID Providers validate the identity of a user for third-party applications. An OpenID Provider is any service/application that provides an OpenID to users. Yahoo, AOL, and others are a few examples of these. Applications that require a user's OpenID to complete transactions are known as OpenID Consumers.
The flow of control in an OpenID Consumer is as follows:
The user is directed to the login page of the supported/selected OpenID Provider.
Once the user completes logging in, the OpenID Provider informs the user about user-related data requested by the OpenID Consumer.
If the user agrees to share the information, he or she is redirected to the page requested by him or her on the consumer application. The information is added to the request URL. The information is termed as attribute properties and this is documented at http://openid.net/specs/openid-attribute-properties-list-1_0-01.html.