The directory service is a collection of location and role information on each staff member/student. It contains information like phone, email and (work) postal address as well as providing a central username/password authentication service.
The messaging server provides location and platform independant access to email through IMAP. All folders are stored on the server, which can be accessed by any IMAP or POP3 client, as well as through a WWW-interface.
The calendar service is like a souped-up Microsoft Schedule (which I've never used, so don't ask me about it! :-p) Users can maintain their own agendas (including a schedule, to-do list, etc) or open other people's agendas to book appointments, add tasks or review their schedule. Levels of confidentiality are available!
The certificate server generates X.509-based certificates which can then be used for identification (are you really who you say you are?), authentication (you're the authorized user of this service), and encryption (PGP-like).
A more secure, restricted news service (ie. only people who match certain criteria can read/post/etc), based on LDAP authentication and attributes.