is there anybody can talk something about ldap?

what's it, what for?

one document i read says that :
a directory service is a database that tells you where things are. DNS is an example of a type of directory service. As things get bigger, as you have more things to track (like users for example) you need a directory service to consolidate all of this data. OID (Oracle Internet Directory) is an LDAP server built on top of Oracle that provides this functionality.

my question is, if we have database, why we need ldap?
seems to me ldap is a simple version of database.
am i right?