slapd is an LDAP directory server. You can use it to provide a directory service of your own. Your directory can most of the things you might want to put in it. You can connect it to the global LDAP directory service, or run a service all by yourself. Some of slapd's more interesting features and capabilities include:
slapd comes with two different backend databases you can choose from. They are LDBM, a high-performance disk-based database, and SHELL, a database interface to arbitrary UNIX commands or shell scripts.
slapd provides a rich and powerful access control facility, allowing you to control access to the information in your database(s). You can control access to entries based on LDAP authentication information, IP address, domain name and other criteria.
slapd is threaded for high performance. A single multi-threaded slapd process handles all incoming requests, reducing the amount of system overhead required.
slapd can be configured to maintain replica copies of its database. This master/slave replication scheme is vital in high-volume environments where a single slapd does not provide the necessary availability or reliability.
slapd is highly configurable through a single configuration file which allows you to change anything you would want to change. Configuration options have reasonable defaults, making your job much easier.
LDAP Version 3 solves some of the problems demonstrated by Version 2. It has improved internationalization support, strong authentication, and other desirable X.500-like features. Note that LDAP Version 3 has been designed to be backwards-compatible with Version 2.
slapd has limitations. It does not currently handle aliases, which are part of the LDAP model. The main LDBM database backend does not handle range queries or negation queries very well.