The Cambridge University Science Fiction Society (CUSFS - pronounced "cuss-fuss") was established in 1963 with the goal of creating a consolidated library of books and magazines. Since then, the society has developed into a catch-all for things related to science fiction, fantasy, and other genres - like alternate history - which bear a spiritual relationship to SFF. If it is in any way relevant, we have probably discussed it, at some point or another.
That 'fantasy' in the last paragraph bears a little explanation. Despite the official name, we are very much the science fiction and fantasy society, but this is a more recent development. At some point - perhaps in 1974, or thereabouts - several members of CUSFS, in protest to what they saw as politicisation of the society (it being, at that time, involved in some sort of CUSU related campaign), formed Jómsborg the New, a separate society which developed into the de facto fantasy society. Some twenty years later, the radical elements of CUSFS having died down, the two societies noticed they had virtually identical member lists and merged.
What does this mean in practice? Nothing, really. CUSFS is the overarching organisational structure; within it, Jómsborg retains a nominal independence. It has its own, largely ceremonial committee; occasional meetings of the society are designated Jómsmeetings, with a fantasy-themed topic and the consumption of mead; and there are a couple of traditional Jómsborg annual events. But in the grand scheme of things, CUSFS is a single society with a unified membership - if, admittedly, a membership that isn't quite willing to dispense with past traditions.
A Fascinating record of the founding and early history of the Society can be read here. Our Archive -- essentially the Society site as it stood before 2013 -- has lots of interesting info about the Society's history, though it's not very well organised (sorry...).