X.500 is an ITU-T set of standards for directory services. The X.500 series originally used the ISO/OSI protocol stack.
X.500 is similar to LDAP, in so far as each of these standards uses a hierarchical directory tree. This hierarchy is expressed through the distinguished name for each object.
The X.509 digital certificate standard originally incorporated X.500, but in current practice, the X.509 standard has been adapted by the IETF for Internet use.
See: RFC 5280
Reference: http://docs.oracle.com/javase/jndi/tutorial/ldap/models/x500.html