OpenDirectory is Apple’s directory and authentication services architecture. It is designed to enable easy integration with a wide variety of technologies such as LdAp, NetInfo, ActiveDirectory, and more. The Directory Services API provides an abstraction layer which can be used to access these various systems, which are all implemented as plug-ins to the framework.
See http://www.apple.com/server/macosx/open_directory.html for a more in-depth overview from the user/administrator point of view.
OpenDirectory is part of Darwin and open-source: http://developer.apple.com/darwin/projects/opendirectory/
The API is an unruly beast. Documentation is available at http://developer.apple.com/documentation/Networking/Conceptual/Open_Directory/index.html and a SDK containing sample source code is available from ftp://ftp.apple.com/developer/Development_Kits/DirectoryServicesSDKv1.0.dmg.bin
A simple Cocoa wrapper for the Directory Services for user authentication and to get basic user info is available for download from http://www.martinkahr.com/source-code