Like glasses, contact lenses work by bending light rays just the right amount to make the object come into focus on the retina at the back of the eye. Sometimes the eye's natural lens has an imperfection or may not be shaped properly, which can make your vision blurry or distorted. Depending on your vision, the right kind of lens helps your eye focus light properly and project a clear image onto the retina.
Glasses have what's called an optical center, which means that objects appear clearest when viewed through the center of the lenses. But when you're wearing glasses and you move your eyes to the side, or up, or down, things can seem slightly distorted or blurry.
So because contacts are right on the eye and move with the eye, there's no optical center.
Put another way, the entire contact lens is the optical center. So no matter which way you move your eyes, objects appear in perfect focus.
"Contact Lenses" (HowStuffWorks)