There are 3 interfaces for accessing the Active Directory:

1) LDAP: The Lightweight Directory Access Protocol (LDAP) is the service protocol that runs on a layer above the TCP/IP layer (or stack) and provides an interface for accessing, searching and modifying Internet Directories, and is based on a client-server model.

2) ADSI: The Active Directory Services Interface (ADSI) is a set of COM components (or Interfaces) designed to access the directory services from different network providers in a network setup, it is designed to provide a single, central interface for accessing & managing network resources.

3) System.DirectoryServices: The System.DirectoryServices Namespace is built into the .Net Framework designed to provide programming access to LDAP directories (Active Directory) and is built on the ASDI API.