ldb is a LDAP-like embedded database. It is not at all LDAP standards compliant, so if you want a standards compliant database then please see the excellent OpenLDAP project.

What ldb does is provide a fast database with an LDAP-like API designed to be used within an application. In some ways it can be seen as a intermediate solution between key-value pair databases and a real LDAP database.

ldb is the database engine used in Samba4.