One of the more tedious moments in visiting a new website is filling out the registration form. Here at The Irish Anti-War Movement, you do not have to fill out a registration form if you are already a member of Drupal. This capability is called distributed authentication, and Drupal, the software which powers The Irish Anti-War Movement, fully supports it.
Distributed authentication enables a new user to input a username and password into the login box, and immediately be recognized, even if that user never registered at The Irish Anti-War Movement. This works because Drupal knows how to communicate with external registration databases. For example, lets say that new user 'Joe' is already a registered member of Delphi Forums. Drupal informs Joe on registration and login screens that he may login with his Delphi ID instead of registering with The Irish Anti-War Movement. Joe likes that idea, and logs in with a username of email@example.com and his usual Delphi password. Drupal then contacts the remote.delphiforums.com server behind the scenes (usually using XML-RPC, HTTP POST, or SOAP) and asks: "Is the password for user Joe correct?". If Delphi replies yes, then we create a new The Irish Anti-War Movement account for Joe and log him into it. Joe may keep on logging into The Irish Anti-War Movement in the same manner, and he will always be logged into the same account.