1. A directory service, LDAP (Lightweight Directory Access Protocol) is used by e-mail programs (Microsoft, Netscape, Eudora, etc.) to allow you to lookup a person's name in a corporate database and find their e-mail address, phone number, and other information that the corporate administrators decided to put in there. Key point: Most corporate LDAP servers have little or no authentication. Finding LDAP servers and downloading their contents is an important step in the reconnaissance phase of a hacking attack. Key point: While LDAP is in theory lightweight, in practice it is still fairly complicated. There are many implementation and deployment bugs that can be exploited in order to break into servers. From Hacking-Lexicon

2. LDAP stands for Lightweight Directory Access Protocol. As the name suggests, it is a lightweight client-server protocol for accessing directory services, specifically X.500-based directory services. LDAP runs over TCP/IP or other connection oriented transfer services. LDAP is defined in RFC2251 "The Lightweight Directory Access Protocol (v3). A directory is similar to a database, but tends to contain more descriptive, attribute-based information. The information in a directory is generally read much more often than it is written. Directories are tuned to give quick-response to high-volume lookup or search operations. They may have the ability to replicate information widely in order to increase availability and reliability, while reducing response time. When directory information is replicated, temporary inconsistencies between the replicas may be OK, as long as they get in sync eventually. There are many different ways to provide a directory service. Different methods allow different kinds of information to be stored in the directory, place different requirements on how that information can be referenced, queried and updated, how it is protected from unauthorized access, etc. Some directory services are local, providing service to a restricted context (e.g., the finger service on a single machine). Other services are global, providing service to a much broader context. From LDAP-Linux-HOWTO

3. Lightwight Directory Access Protocol. Allows network nodes to get info on other nodes. From Linux Guide @FirstLinux