The Microsoft Active Directory Management Gateway Service lets information technology professionals manage Active Directory Directory Service and Active Directory Lightweight Directory Service instances that are running on the same server. The Active Directory Management Gateway Service is available as part of Windows Server 2008 R2 and available as a separate download for some previous versions of Windows Server 2008 and Windows Server 2003.

Information technology professionals can use Active Directory Module for Windows PowerShell or Microsoft Active Directory Administrative Center (ADAC) to manage Active Directory Management Gateway Service instances on servers that are running Windows Server 2008 Service Pack 1 (SP1) and later versions and Windows Server 2003 Service Pack 2 (SP2) and later versions.

The Active Directory Management Gateway Service provides the same functionality as Active Directory Web Services on Windows 7. After you install the Active Directory Management Gateway Service, the service runs as the Windows Server R2 Active Directory Web Services service.

http://support.microsoft.com/kb/969041