What is LDAP?
I am not going to define LDAP very deeply. Simply, it is a lightweight version of Directory Access Protocol (DAP), which is a part of X.500, a standard for directory service in a network.
Overview
LDAP can be used for different purposes. Some people use it for maintaining their centralized email address book, for building the domains, for authentication of users from centralized database, for querying and modifying the other directory services etc.