About the college thing ... there are 30 odd colleges in Cambridge, many of them right in the centre of the city, others of them a little bit out of town (or, in the case of Girton and Homerton, a healthy cycle ride out of town). You become a member of your college as soon as you arrive and "matriculate"; but you don't become a member of the university until you graduate.
Your college is your "base". It is where you will live for at least two of your years of study, and at some colleges you can "live in" for all three years. At Magdalene in my day (rather a long time ago) only a third of second year undergraduates had rooms in college; but I was one of the lucky ones. At most colleges you can choose either to prepare your own meals or to eat in the communal dining rooms with your fellow students. All colleges have "formal hall" evening meals. At most college this is not available every night; but at a few it is. These are wonderful occasions and build a great sense of community.
There are sporting and social clubs and societies within each college, as well as university societies. Your social life is likely to revolve around a mixture of college and university societies.
Lectures and examinations are organised by the university. Supervisions are organised by your college. Supervisions involve a small group of students (between 1 and 6, but most usually 3 or 4) having a 1 hour seminar with a senior academic in their subject. They will not all be in your own college, but many of them will. The other students in your seminars will generally be from your college, but there will be times when you attend mixed supervisions, with students from other colleges.
Different colleges have different traditions relating to a large number of aspects of college life. These are constantly evolving, however. You become part of your college; you inherit the traditions which have been shaped by those who have studied there before you, and you will add your own contribution before passing them on to those who come after you.
It is very true that the different colleges have more things in common that they have things that distinguish them from one another; but the distinctions are nevertheless important. So if you are able, it is well worth taking the time to visit the colleges that you are interested in (you mention Catz, for instance ... right in the centre of town on King's Parade. Tourist central. Some people love that, others hate it. Which are you? Girton, on the other hand, scarcely sees a tourist all year, but is a long ride out of town. WIll you relish that, or will you hate puffing up and down Castle Hill twice daily?). Some colleges are very sporty, and the college social life is dominated by the rugby and boat clubs; others are less so. They all get pretty excited about the Bumps, however ...
Cambridge is a wonderful, intense, life-changing experience. Very few who have won a place there ever look back on it with anything but the fondest recollection. It may be a damp little town on the edge of the Fens ... but WHAT a town; and WHAT a university! (And they never built cars there, either ... )