LDAP works as a client and a server. Most application servers support LDAP at some level, and others are available for free. If you write your code to use the JNDI-compliant interfaces, then you should be able to switch implementations without a problem.

The required files vary greatly from provider to provider. Your application server, if it contains an LDAP implementation, should give you the provider code to use with your application. A third-party product should do the same.

Try looking at O'Reilly's Java Enterprise In A Nutshell or any other book containing material on JNDI or LDAP.

-chris