Active Directory Service Interfaces (ADSI) is a programmatic
interface for the Microsoft Windows Active Directory. It
enables your applications to interact with different
directories on a network using a single interface.

Name space
-----------
System.DirectoryServices

Active Directory Service Interfaces (ADSI) is a set of COM 
interfaces used to access the features of directory 
services from different network providers. ADSI is used in 
a distributed computing environment to present a single set 
of directory service interfaces for managing network 
resources.

Namespace :
System.DirectoryServices