Argon and krypton are odorless, colorless, non-toxic inert gases that can be used instead of air between panes of glass to increase insulation and energy efficiency. Argon is the cheaper, more readily available gas, but Krypton is a better insulator. Often, manufacturers will use a blend of air, argon, and/or krypton to balance cost and performance.
When purchasing energy efficient windows, doors, and skylights, it is more important to consider whole-unit U-factor (a measure of insulation) and Solar Heat Gain Coefficients (SHGCs) (a measure of how well the product blocks heat caused by the sun) rather than the specific materials or technologies used. The ideal U-factor and SHGC vary depending on the product and/or your climate zone.