In the case of collegiate universities such as Oxford and Cambridge, a college is an autonomous body which is distinct from the university. As far as the student is concerned, the university provides teaching in the form of lectures and examinations (and lab work etc.) and the college provides accomodation, meals and teaching in the form of supervisions (or tutorials, depending on where you're from.) and possibly examples classes. The university awards your degree.

Cambridge, for example, currently has 31 colleges, and although the prospectus will tell you that they are more alike than different, if you are currently deciding which to apply to here are a few things to consider:

  • Age: ranges from thirteenth to twentieth centuries, and will affect the architecture more than the facilites. Peterhouse is old.
  • Money: some colleges are very, very rich, and this is reflected in the grandure of the buildings, the size of the library and the scholarships and travel grants available. Trinity is rich.
  • Academic stature: usually tied to the above, note that you may be intimidated working in the presence of very gifted people. People from Trinity are also clever bastards.
  • Size: ranges from about 200-1000 undergraduates. Bearing in mind that most of the people you'll socialise with will be from your college, you might want to give this a thought. Corpus Christi is small.
  • Quality of the May Ball: even if you don't want to go, you could make a bit from flogging your tickets. John's has a massive May Ball.
  • Distance from town: Not usually a great factor, although some colleges are a long way from the town centre. Girton is miles away.