What's the LDAP?

The Lightweight Directory Access Protocol --commonly referred to simply as LDAP -- is the most widely used network protocol used to access a directory of
users,computers, groups, etc.

LDAP is normally used by applications to access a directory that contains the identity ofusers
who can sign in. Having the application connect to the directory server using the user's login ID and password is often used to authenticate users.Testinga
user's membership in securitygroups in the directory or using an attribute
based access control policy based on identity attributes in the directory are common strategies for authorizing user
actions within an application.

It is an application protocol used over an IP network to manage and access the distributed directory information service .

LDAP directories are structured as a tree of entries, where each entry consists of a set of attribute-value pairs describing one object. The objects are often people,organisations, and departments,
but can be anything at all. Schemais the term used to describe the shape of the directory and the rules that govern its content.

For more on LDAP server andLDAP data structure it very detailed and somewhat advanced paper on LDAP

ldap-schema-design:
http://www.skills-1st.co.uk/papers/ldap-schema-design-feb-2005/ldap-schema-design-feb-2005.html