Someone who shares a dorm room or an apartment with you is your roommate. Even if you live in a big house, the people who share it with you — and split the rent — are your roommates.
If you live in a dormitory when you go to college, you'll probably have at least one roommate, and even after college it's common to share an apartment or house with roommates. You can call the campers in your cabin at summer camp roommates, though you could also call them cabin mates. The word roommate was a late eighteenth century American English invention.