You can use any LDAP compatible directory services. Java Naming and Directory Interface (JNDI) is a standard extension to LDAP.
Servers, which are built on LDAP, are optimized for read access so likely to be much faster than RDBMS in providing read access. So in a nutshell, LDAP is more useful when the information is often searched but rarely modified. (Another difference is that RDBMS systems store information in rows of tables whereas LDAP uses object oriented hierarchies of entries.) .