There is a wire bead in the tire where it seats into the rim. Pricier tires often have a bead made of kevlar or some other lighter high tech material.

My hybridized MTB has Panaracer Paselas with a wire bead. Good all around street tires, reasonably light. They also make a more flat resistant version, the Pasela TG (Tour Guard), which is just a few grams heavier.