Microsoft Knowledge Base Article 912492 contains the following summary and introduction:
Active Directory Federation Services (ADFS) enables web single sign-on, federated identity and access management by securely sharing digital identity
and entitlement rights across security and enterprise boundaries. The ADFS functionality becomes available in Windows Server 2003 R2.
The purpose of this article is to describe supported configurations, known limitations, and recommendations for improving the user experience when
you deploy Microsoft Windows SharePoint Services and Microsoft Office SharePoint Portal Server 2003 with ADFS.