^^^^Good link^^^^
Its important to note that the first number (width) is the section width of the tire, not the treadwidth. Its the number of mm at the widest point of the tire, which is almost never at the tread.
In general, touring, light truck, and passenger tires have very narrow treadwidths compared to the section width giving it a donut or bulging look. Performance tires typically have wider treads giving it that flat sidewall look.
The tire manufacturers list these parameters on their websites, like this one from goodyear;
http://www.goodyeartires.com/goodyea...area=Passenger