OpenLDAP is a robust Lightweight Directory Access Protocol (LDAP) implementation for Linux that ships with most Linux distributions. LDAP is very similar to Active Directory in that it's a database. However, LDAP is more of a read-only (or write-little) database, which differentiates it from other database types, such as MySQL.
A common use of the LDAP protocol is for authentication purposes. In larger networks, where many people use multiple machines, user management can be difficult. If users change their passwords on one machine, they have to change them on all the other machines they use. With LDAP, you can change your password on one system, and it reflects this change on all other machines that use LDAP for authentication.
Since LDAP is a standard, you can use it with multiple operating systems. If you use Samba, you can handle user authentication and password management on Windows clients. When you tie Samba to LDAP, these tasks are possible across multiple Samba servers as well. Similarly, you can use LDAP for authentication on Linux, BSD, and other UNIX systems by using pam_ldap. Even Mac OS X supports LDAP as an authentication mechanism.
To implement LDAP authentication, follow the step-by-step instructions that are available from the Mandrake Web site.