Many directory services exist, such as Novell Directory Services, DCE Cell Directory Service, Banyan StreetTalk, Windows Directory Services, X.500, and the address book services associated with e-mail products. X.500 was proposed as a standard for global directory services by the International Standards Organization (ISO). It requires an OSI protocol stack for its communications, and largely because of this, its use has been restricted to large organizations and academic institutions. An X.500 directory server communicates with its clients using the Directory Access Protocol (DAP).
LDAP (Lightweight Directory Access Protocol) was created as a simplified version of DAP. It is easier to implement, omits some of the lesser-used features of DAP, and runs over TCP/IP. As a result of these changes it is rapidly being adopted as the directory access protocol for most purposes, replacing the multitude of proprietary protocols previously used. LDAP clients can still access an X.500 server through a gateway (X.500 still requires the OSI protocol stack), or increasingly X.500 implementations typically include native support for LDAP as well as DAP access.
LDAP directories can be distributed and can use replication to enable efficient access to their contents.
For a more complete description of LDAP, please see IBM Redbook Understanding LDAP.