The Lightweight Directory Access Protocol (LDAP) is an open, standards-based, mature, efficient, extensible, and popular means of interacting with the data contained in directory servers. It is a binary (i.e., not text-based) protocol that uses TCP/IP for transport. The standard TCP ports used by LDAP are 389 for unencrypted communication and 636 for communication encrypted by SSL/TLS, although it is not uncommon for LDAP servers to use alternate ports for various reasons.
The most recent version of the protocol is LDAPv3, and the wire format for LDAPv3 is described in RFC 4511. See the LDAP Specs page for links to a full set of LDAP specifications and other reference materials.
LDAP is a fairly general-purpose data access protocol, so it can be used in a very wide range of applications. LDAP can be used to create, delete, alter, retrieve, search for, and retrieve just about any kind of information. Although there are certainly some applications for which LDAP is not ideally suited (e.g., there is no standard LDAP mechanism for streaming data, so it is probably not a good choice for huge multimedia clips or other objects that are better retrieved in segments rather than all at once), the uses for LDAP as a general means of interacting with data are unlimited.
It also provides strong support for authentication, whether via simple password or potentially through other mechanisms, which is not something that is as prominent in other means of interacting with data like SQL for relational databases and most NoSQL implementations. This makes it a very good choice for any application which needs to authenticate users, and as a result, LDAP directory servers are often used for storing user accounts and related information.
Another key use of LDAP in storing and interacting with name services information. Most operating systems provide at least some level of support for storing and interacting with about users, groups, services, hosts, networks, etc. using LDAP. LDAP has become the de facto replacement for older name service protocols like NIS.
The following pages provide much more information about the LDAP protocol, directory servers, and related information: