[andrew1]

Only some UK universities have a collegiate system (Oxford, Cambridge, Durham, St. Andrews(?)). Most do not (I believe) and I imagine are roughly similar to US universities. At collegiate universities there is a split between what a college is responsible for, and what the university is responsible for. Lectures are university-wide, exams also are (you get your degree from the university not the college), departments belong to the university (so if the Enlgish department needs a new building then the university pays for that). All students (and staff) are affiliated to a college (you choose which college to apply to when you apply to the university, and the college decides whether to admit you). The college is responsible for their students accommodation needs, will usually provide a dining option, common room etc. The college provides teaching (so you go to the same lectures as everyone else, but your small group teaching, feedback on work etc. is organised by the college). I think those are the main distinctions.

Given that the colleges are different, this can mean it's important to choose the right one for you. For example, at Cambridge, Trinity College has an illustrious history in mathematics, and attracts excellent staff, so if you're accepted there then you might end up being taught one-on-two by a world renowned mathematician (Fields Medal winning Timothy Gowers for example). Other colleges with less of a reputation might have more trouble attracting the same type of staff. (this isn't to say that being an expert in your field makes you an excellent teacher of course...)

[sachinag]

OK, this really helps; thanks! Let's say that you really wanted to study CS at Oxford; if you pick the wrong college to apply to, are you shut out from the university entirely? Or can you apply to multiple colleges at the same university at once?

[ig1]

If a college thinks you're good at your subject but doesn't want to accept you for other reasons (they have too many candidates, don't think you'd be a good fit) they can "pool" you, which means that other colleges that are interested can interview you.

You can also make an "open application" which is similar to pooling where you say you don't mind which college you get into.

Generally applicants tend to put too much emphasis on the "right" college, the academic differences aren't that great, and a lot of it is just candidates wanting to get into the more prestigious colleges for prestige's sake.

[chrisaycock]

It's different for postgraduates. Undergrads can only apply to one college and to one uni at a time, as @andrew1 mentions. Postgrads can apply to both unis. As for college selection, I had to apply to my department first (ComLab) and then I listed four colleges in order of preference. Once the department accepted my application, it was up to one of the colleges to accept me, though I did get my first choice.

[andrew1]

Nope, you can only apply to one college at either Oxford or Cambridge per year (you can't apply to both universities, which probably makes sense as almost everyone who applied to one would probably apply to the other). There is a clearing system I think where if the college you apply to doesn't want you then other colleges can look at your application and choose to interview you, but I think that's fairly rare.

[flipbrad]

Whilst you're discouraged at all levels from applying to both Ox and Cam, I don't think it's true that you can't. But you're correct about the second interviews (at other colleges). This was actually pretty standard (for Biochem, at Oxford) when I applied and you went for the alternative interview (at a college you're assigned to, you don't get a second choice) before the first has made any sort of decision on you. There is a second safety net, whereby you can be 'pooled' (which is basically what you describe); if you don't have a college preference from the outset, you can apply directly to the pool (again, this is Oxford, not necessarily Cambridge)

[ig1]

You can't, UCAS won't accept your application. There are a few exceptions where you are allowed to apply for both, but in general you can't.