What Is ADSI?
ADSI is a set of open interfaces that abstract the capabilities of directory services from different network providers to present a single view for accessing and managing network resources. Administrators and developers can use ADSI services to enumerate and manage resources in a directory service, no matter which network environment contains the resource. This can be an LDAP-based, NDS-based, or NTDS-based directory. It does not matter so long as a service provider is available for that directory service.
From http://www.microsoft.com/ntserver