Skylights

Tempered, double-insulated glass.

This is the minimum standard for most quality products: tempered for safety and two layers of glass with an air space in-between for energy efficiency
Tempered, double-insulated glass with Low-E (low-emissivity) coatings.

With this glass, the outer pane is tempered, and the inner pane has an invisible low-e coating that helps hold heat in during the winter, reduces heat gain in the summer, and cuts ultraviolet (UV) rays, which bleach fabrics and damage furniture and floors.
Tempered, Low-E, argon-filled glass with a bronze tint.

This features the same qualities as the above two but is designed to cut back on solar heat gain.
UV blockage

UV blockage is a value approximated by glazing manufacturers, expressed as a percentage of ultraviolet rays blocked by the glazing.