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they represent the tire size. If you want to know what tire size to get for winter, well, you are restricted by your wheel size unless you are getting new wheels.
Check out tirerack.com. Just punch in your vehicle's info and what you're looking to get, in this case Winter Tires. They also have packaged deals where you can get both tires and wheels at a fairly reasonable price.
I just bought some 17" Dunlop M3 along w/ some cheapo wheels even though my stock wheels and tires are 18". Don't want anything too nice that'll get eaten up by the winter road salt.
Good luck!