A quick rundown:
The student unions (e.g. http://en.wikipedia.org/wiki/Student_unionism_in_Finland ) are part of the university administration, have 100's of millions in assets and provide basic services (healthcare, apartments, places to party) in exchange for 40+ euros per year.
The subject- and interest-based associations are separate entities that get some funding from the student union and organize events. They don't charge, and mostly get their cash from organizing events. However, since everybody lives around town, the interactions are more focused around specific events. The nations (http://en.wikipedia.org/wiki/Nation_(university) ) are a lot like the student associations, except they are specific to people from a particular area in the country (not strictly enforced), have a bit more money (e.g. may have their own apartments) and organize activities centered around traditions and whatever is popular.
Thanks for a good question, interesting to see the parallels in other countries...