Check the beginner docs at
http://www.ldapman.org to get started.
LDAP is a protocal, like HTTP, and as such it relies on LDAP servers. In the same way that Apache and IIS manage HTTP communication, servers like IBM SecureWay, Netscape's NDS and OpenLDAP are LDAP implementations.
There are many things that LDAP does, but in its most basic usage, it is used as a heirarchical directory listing and if often compared to a phone book.
It can also manage security and role mappings and vertical and horizontal replication. The difference between LDAP directories and relational databases is that LDAP directories are typically optimised for read and search operations, not create, update or deletes.
Dave
