Apple Open Directory

Apple Open Directory is the a directory service which stores and organizes information about a computer network's users and network resources and which allows network administrators to manage users access to the resources. 

Open Directory began with Mac OS X Server 10.2. In this initial form, Open Directory consisted of a network-visible NetInfo directory domain and a corresponding Authentication Manager service for storing passwords outside of the directory. Version 10.2 also included support for Kerberos. Mac OS X versions 10.1 and 10.0 stored user password information within the directory domain using crypt password authentication authorities, but version 10.2 paved the way for the current Shadow Hash and Password Server mechanisms. 

In a more general sense, Open Directory can describe the plugins model used by Directory Utility and the directory services framework in Mac OS X and Mac OS X Server. This could be thought of as analogous to the Name Service Switch systems of some other Unix-like operating systems.

Here is the link to access https://ssl.apple.com/support/macosxleopardserver/opendirectory/