Microsoft Knowledge Base Article 307900 contains the following summary:

Windows XP Professional supports a superset of the Group Policy features supported by Windows 2000. The new features are implemented through updated client-side extensions, .adm files, and an updated Group Policy snap-in.

This article describes how to upgrade a Windows 2000 Active Directory Group Policy object (GPO) to support the new features in Windows XP, and how to manage such objects.