The Lightweight Directory Access Protocol (LDAP) is a protocol for accessing online directory services. It runs directly over TCP and can be used to access a standalone LDAP directory service or a directory service maintained by X.500. A directory is type of database that stores information that is read more often that written to and so services are typically geared for high volume read access and offer simpler transaction support than general purpose databases. The LDAP directory service model is based on entries. An entry is a collection of attributes that has a "distinguished name" (DN). The DN is used to refer 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 email address. The values depend on what type of attribute it is: an email attribute might contain the string value "foo@bar.org". A jpegPhoto attribute would contain a photograph in binary JPEG format.

LDAP directory entries are arranged in a hierarchical tree-like structure that may reflect political, geographic and/or organizational boundaries. Hence, entries representing countries appear at the top of the tree, below them are entries representing states or national organizations, then, entries representing people, printers, documents, anything else...

LDAP provides methods for authentication. Directories can be made accessible to the general public or protected however the case may be.

LDAP is made for finding people and resources on a network. It provides a completely different functionality from web searches such as yahoo or webcrawler, which work simply by text matching and often return many thousand's of entries. Using an LDAP directory to locate something however, if you know the approximate location of where it is, such as what organization and country it is in, then you can do a search and return much fewer entries. LDAP is a vendor independent open protocol. The fact that organizations can alter it to their own needs is key. Also, large companies can use it as the basis for their own more complex directory servers.

There are common misconceptions about the role of LDAP. LDAP is not intended as a replacement for local databases. They are not built to be added and modified too easily and thus would not work in a situation such as an airline reservation system. Also, LDAP is not meant to be a replacement for DNS. DNS is a specialized well used service on the Internet for matching IP addresses to real names. However databases and DNS and LDAP like most Internet services work very well in collaboration.

Virtuoso has the ability to act as an LDAP client, using built-in functions, and has been tested as a back end for the OpenLDAP server.