Lightweight Directory Access Protocol (LDAP) is a protocol for accessing on-line directory services.
LDAP was defined by the IETF as a better way to make use of X.500 directories. The Directory Access Protocol (DAP) was seen as too complex for simple internet clients to use. LDAP defines a relatively simple protocol for updating and searching directories running over TCP-IP.
The common term "LDAP directory" is misleading. There is no specific type of directory that is as an "LDAP directory". It is fair to use the term to describe any directory which is accessible using the LDAP protocol and can identify objects in the directory with X.500 identifiers. There are some directories, such as OpenLDAP, which were primarily designed as repositories for X.500 data using access by LDAP, but that doesn't make them any more of an "LDAP directory" than any other directory accessible by the LDAP protocol.
An LDAP directory entry is a collection of attributes with a name, called a distinguished name (DN). The DN refers to the entry unambiguously.
Each of the entry's attributes has a type and one or more values. The types are typically mnemonic strings, like "cn" for common name, or "mail" for e-mail address.
The values depend on the type. For example, a mail attribute might contain the value "firstname.lastname@example.org". A jpegPhoto attribute would contain a photograph in binary JPEG/JFIF format.
LDAP directory entries are arranged in a hierarchical structure that reflects political, geographic, and/or organisational boundaries. Entries representing countries appear at the top of the tree. Below them are entries representing states or national organisations. Below them might be entries representing people, organisational units, printers, documents, or just about anything else.
RFCs on LDAP
This article (or an earlier version of it) contains material from FOLDOC, used with permission.