Cover your eyes and protect your sight from insects, stones and the sun's UV while out riding.
Cycling glasses are designed to vent heat to reduce the fog that can arise when you stop. This means they don't sit quite so flush with your face.
Look for impact resistant lenses, that wrap around for better peripheral vision. Some have interchangeable lenses to adjust to different conditions, others come with photochromatic lenses. Other styles are suitable for those with prescription lenses.
Look for lenses with a UV rating to ensure you have UV protection.