Citadel is a "turnkey open-source solution for email and collaboration". the main component is the citadel server. to communicate with it you can use either a web interface, or - if you have to automate things - with a protocol. net::citadel tries to do a bit of abstraction and handles some of the protocol handling. the basic idea is that the application using the package deals with citadel's objects: rooms, floors, users.