Most lenses are either glass, plastic, or polycarbonate. Glass lenses provide good scratch resistance, withstand chemicals, and allow for prescriptions. But glass is heavy and can be uncomfortable. Plastic and polycarbonate lenses are light weight, are less likely to fog, and protect against welding splatter. These lenses are not as scratch resistant as glass.
Polycarbonate lenses are superior to glass and plastics in strength and impact resistance.
Lenses come in different colors. Clear lenses are for most general applications. Amber or orange lenses increase vision and clarity in low light conditions. Gray and green tints, and mirrored lenses provide protection in bright light or flash environments. Polycarbonate lenses provide protection from standard UV radiation.