Lectures are organized by the relevant subject's department, independently of the colleges; in many ways the departments are as autonomous as the colleges. But supervisions, the more important part of the Cambridge style of teaching, are organized by your college (though smaller colleges with only a few students in a given subject will often share supervisions, and even the biggest colleges might send you elsewhere to be supervised if you've picked an obscure specialization in your third year). Almost all colleges offer almost all subjects (there are a few special cases like st edmund's which is a dedicated theological college AIUI), but some do have a reputation for particular subjects; many (but by no means all) computer science students (in general most students don't have a minor in the UK, especially in Cambridge) go to either Trinity Hall (for its history/reputation and its generous internet usage policies) or to Churchill (which is physically closest to the Computer Science department)