Single Sign-on Overview
Microsoft Office SharePoint Portal Server 2003 included a single sign-on (SSO) service for storing and mapping user credentials for use with third-party, back-end server applications. In Microsoft Office SharePoint Server 2007, this functionality is enhanced to include a pluggable single sign-on mechanism that allows you to specify alternate single sign-on providers.
Replacing the default SSO provider in Office SharePoint Server 2007 involves implementing the Microsoft.SharePoint.Portal.SingleSignon.ISsoProvider class, installing it into the global assembly cache, and registering the new SSO provider with Office SharePoint Server 2007.