ADSI (Active Directory Service Interfaces)

ADSI abstracts the functionality of different directory services from various network vendors into a single set of directory service interfaces for managing network resources. Administrators and developers can use ADSI to manage the directory service resources regardless of where on the network the resource is located. ADSI enables administrators to automate frequent tasks such as managing printers, managing users and groups, and setting network resource permissions. ADSI offers developers access to multiple directory service providers through a set of open interfaces. Using ADSI, applications are written to work with any directory service that offers an ADSI provider—Active Directory, LDAP, NDS, etc. The .NET managed version of ADSI is System.DirectoryServices

Reference: http://msdn2.microsoft.com/en-us/library/aa772170.aspx