Active Directory is simply defined as the meta data 
collection for users and objects and their configuration 
along with their management. Active Directory in Windows 
2000 utilizes the Domain Naming Service (DNS) standard for 
naming objects.

Active Directory Service Interfaces (ADSI) is a programmatic
interface for Microsoft Windows Active Directory. It enables
your applications to interact with diverse directories on a
network, using a single interface. Visual Studio .NET and
the .NET Framework make it easy to add ADSI functionality
with the DirectoryEntry and DirectorySearcher components.
Using ADSI, you can create applications that perform common
administrative tasks, such as backing up databases,
accessing printers, and administering user accounts. ADSI
makes it possible for you to:
• Log on once to work with diverse directories. The
DirectoryEntry component class provides username and
password properties that can be entered at runtime and
communicated to the Active Directory object you are binding to.
• Use a single application programming interface (API) to
perform tasks on multiple directory systems by offering the
user a variety of protocols to use. The DirectoryServices
namespace provides the classes to perform most
administrative functions.
• Perform "rich querying" on directory systems. ADSI
technology allows for searching for an object by specifying
two query dialects: SQL and LDAP.
• Access and use a single, hierarchical structure for
administering and maintaining diverse and complicated
network configurations by accessing an Active Directory tree.
• Integrate directory information with databases such as SQL
Server. The DirectoryEntry path may be used as an ADO.NET
connection string provided that it is using the LDAP provider.
using System.DirectoryServices;

Active Directory Service Interfaces (ADSI) is a programmatic
interface for Microsoft Windows Active Directory. It enables
your applications to interact with diverse directories on a
network, using a single interface. Visual Studio .NET and
the .NET Framework make it easy to add ADSI functionality
with the DirectoryEntry and DirectorySearcher components.
Using ADSI, you can create applications that perform common
administrative tasks, such as backing up databases,
accessing printers, and administering user accounts. ADSI
makes it possible for you to:
• Log on once to work with diverse directories. The
DirectoryEntry component class provides username and
password properties that can be entered at runtime and
communicated to the Active Directory object you are binding to.
• Integrate directory information with databases such as SQL
Server. The DirectoryEntry path may be used as an ADO.NET
connection string provided that it is using the LDAP provider.
• Use a single application programming interface (API) to
perform tasks on multiple directory systems by offering the
user a variety of protocols to use. The DirectoryServices
namespace provides the classes to perform most
administrative functions.
• Perform "rich querying" on directory systems. ADSI
technology allows for searching for an object by specifying
two query dialects: SQL and LDAP.
• Access and use a single, hierarchical structure for
administering and maintaining diverse and complicated
network configurations by accessing an Active Directory tree.

using System.DirectoryServices;