What is LDAP?

LDAP is a Lightweight Directory Access Protocol. A directory contains objects; generally those related to users, groups, computers, printers.LDAP gives you query methods to add, update and remove objects within a directory.LDAP is also used to store your credentials in a network security system and retrieve it with your password and decrypted key giving you access to the services.
For example:

use the same login/passwd to login on an Intranet and on your local computer.
give specific permissions to a group of user. For example some could access some specific page of your Intranet, or some specific directories on a shared drive.
get all the contact details of the people in a company on Outlook for example

LDAP was designed at the University of Michigan to adapt a complex enterprise directory system (called X.500) to the modern Internet. X.500 is too complex to support on desktops and over the Internet, so LDAP was created to provide this service “for the rest of us.”

LDAP servers exist at three levels: There are big public servers, large organizational servers at universities and corporations, and smaller LDAP servers for workgroups. Most public servers from around year 2000 have disappeared, although directory.verisign.com exists for looking up X.509 certificates. The idea of publicly listing your email address for the world to see, of course, has been crushed by spam.