A structured repository of information on people and resources within an organisation, facilitating management and communication.

On a LAN or WAN the directory service identifies all aspects of the network including users, software, hardware, and the various rights and policies assigned to each. As a result applications can access information without knowing where a particular resource is physically located, and users interact oblivious to the network topology and protocols.

To allow heterogeneous networks to share directory information the ITU proposed a common structure called X.500. However, its complexity and lack of seamless Internet support led to the development of Lightweight Directory Access Protocol (LDAP) which has continued to evolve under the aegis of the IETF. Despite its name LDAP is too closely linked to X.500 to be "lightweight".

LDAP was adopted by several companies such as Netscape Communications Corporation (Netscape Directory Server) and has become a de facto standard for directory services. Other LDAP compatible offerings include Novell, Inc.'s Novell Directory Services (NDS) and Microsoft Corporation's Active Directory. The Netscape and Novell products are available for Windows NT and Unix platforms. Novell Directory Services also run on Novell platforms. Microsoft Corporation's Active Directory is an integral part of Microsoft's Windows 2000 and although it can interface with directory services running on other systems it is not available for other platforms.