LDAP was originally developed as a front end to X.500, the OSI directory service. X.500 defines the Directory Access Protocol (DAP) for clients to use when contacting directory servers. DAP is a heavyweight protocol that runs over a full OSI stack and requires a significant amount of computing resources to run. LDAP runs directly over TCP and provides most of the functionality of DAP at a much lower cost.
This use of LDAP makes it easy to access the X.500 directory, but still requires a full X.500 service to make data available to the many LDAP clients being developed. As with full X.500 DAP clients, a full X.500 server is a large software system to run.
The stand-alone LDAP daemon, or slapd, is meant to remove much of the burden from the server side just as LDAP itself removed much of the burden from clients. If you are already running an X.500 service and you want to continue to do so, you can probably stop reading this guide, which is all about running LDAP via slapd, without running X.500.