In the UNIX world, directory services like NIS and NIS+ have a long history, but both are fairly tight-coupled with UNIX. Integrating Linux in a NIS environment does not require any modification in the PAM configuration, while the Name Service Switch (NSS) in the GNU C Library embraces NIS for authentication purposes, and the pam_unix module can be used.
The Lightweight Directory Access Protocol (LDAP) is on the other hand a generic framework, and a set of protocols and data format, which can be used to capture any information about an organization—including users and computers.
Many vendors have embraced LDAP, but one particular implementation is very popular—Microsoft Active Directory (AD). AD is Microsoft's own version of LDAP and Kerberos. Prior to AD, Microsoft had its proprietary mechanisms for authenticating users. But even Microsoft is approaching integration with Linux/UNIX by using open standards. Of course, Microsoft adds small deviations to the open standards...