The Common Rooms exist to represent students, to organise social and cultural events and to support clubs and societies. The rooms themselves are relaxed spaces for students to meet and socialise with others.
Undergraduates are members of the Junior Common Room (JCR), and more information can be found on the JCR website. Also, have a look at the student-created Inside Brasenose website.
Graduate students and some undergraduates on four-year courses are members of the Hulme Common Room (HCR). This is the equivalent of the Middle Common Room or MCR in most other colleges. Please see the HCR website for more information.