Citadel is a "turnkey open-source solution for email and collaboration" (this is as far as marketing can go :-). 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
This package tries to do a bit of abstraction (more could be done) 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.