What is LDAP ? A basic idea.
The Lightweight Directory Access Protocol(LDAP) is a client-server protocol for accessing directory protocol. It runs over TCP, typically in 389 port. A directory is an organized set of records. For an example a telephone directory is a set of subscribers with address and telephone numbers. An LDAP directory can be distributed among many servers on a net work.
Following image will help you to get a basic idea how it works.
Further more reference : Wikipedia : http://en.wikipedia.org/wiki/LDAP