LDAP Server

LDAP stands for Lightweight Directory Access Protocol -- LDAP, located in part of the Computers & Internet hierarchy. LDAP is not so popular.
"LDAP is a client-server protocol for accessing a directory service. It was initially used as a front-end to X.500, but can also be used with stand-alone and other kinds of directory servers."
LDAP lets you "locate organizations, individuals, and other resources such as files and devices in a network, whether on the Internet or on a corporate intranet," and whether or not you know the domain name, IP address, or geographic whereabouts. An LDAP directory can be distributed among many servers on a network, then replicated and synchronized regularly. An LDAP server is also known as a Directory System Agent (DSA). LDAP is generally used for reading the information.
LDAP was developed at the University of Michigan; it's "lightweight" in contrast to DAP, a part of the older X.500 directory protocol for networks.
It is both simpler and less secure, hence lighter. It can be used to route email in large organizations as well as look up people and machines across public or private networks.
Many current email clients, including Microsoft Outlook, Eudora, and Netscape Communicator, use some form of LDAP database to look up email addresses. Internic and Infospace are two examples of big public look-up services built with LDAP.