Cycling glasses typically wrap close to your face to keep wind from getting behind the lenses, which can really irritate your eyes. Ideally, they'll also do a better job blocking light around your peripheral vision (especially at the top) than, say, a pair of Wayfarers.The lenses are usually made out of polycarbonate plastic, which is less likely to shatter. Admittedly, polycarbonate is pretty common with sunglasses in general these days.The very earliest Oakleys (which I think of as the progenitors of the whole cycling-glasses genre) even had a foam pad along the upper edge to keep sweat out of your eyes.
Cycling glasses are fairly similar, if not identical, to other sports sunglasses. But they are significantly different than fashion sunglasses.
When comparing general sports sunglasses to cycling-specific sunglasses, I've noticed a few trends:
More of the cycling-specific models have interchangeable lenses than general sports models.
More of the cycling-specific models have vents.
Hydrophobic coatings seem to be harder to find on cycling-specific models.
These are just my observations based on statistically insignificant sample sizes.