[lamilein]

taken from the drupal page, describes very good what we need:

"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 drupal.org. 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 drupal.org. Joe likes that idea, and logs in with a username of joe@remote.delphiforums.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 drupal.org account for Joe and log him into it. Joe may keep on logging into drupal.org in the same manner, and he will always be logged into the same account."