It is your dorm, but it's more than that. Your college stays with you your entire time at Rice.
If you live on campus, you live in your college (there are rare exceptions, usually relating to a college having a couple vacancies and another college having students who got booted off campus but want to be on). If you play team IM sports, you play for your college team. Beer Bike is a race of the colleges.
You graduate in alphabetical order within your college with your college masters calling the names for each college.
Colleges will put on plays, parties (both exclusive and campus-wide), even sponsor off-the-beaten-path courses (some apply for and get an hour or two of credit for those courses).
We've got a group of ~40 from my college (Brown) that gets back together 2-3 times a year (usually for 4th of July or a March ski trip + whatever weddings are going on for group members). Our gatherings tend to be anywhere from 10-25 of us. The last was Memorial Day weekend and the next is July 4th, followed by a wedding in August. This has been going on for over a decade now.