LDAP, the Lightweight Directory Access Protocol, is a communication protocol for directory applications defined in RFC 1777.
Quoted from RFC 1777:
[LDAP] is designed to provide access to the X.500 Directory while not incurring the resource requirements of the Directory Access Protocol (DAP). This protocol is specifically targeted at simple management applications and browser applications that provide simple read/write interactive access to the X.500 Directory, and is intended to be a complement to the DAP itself.
LDAP servers usually store (but are not limited to) information about people such as their name, phone number, email address, office location, etc... More information about LDAP can be found at http://www.openldap.org/.
EUDC requires external support to access LDAP directory servers (see LDAP Requirements)