Glazing system types affect energy efficiency and the amount of light which can pass through the window. Standard single-pane glass has very little insulating value (the R-value is approximately R-1, the U-value is about 1.1) and is usually responsible for significant heat loss and gain. Generally considered very energy wasteful.

Until fairly recently, improving a window's energy efficiency meant increasing the number of glass panes, because each layer of glass with air between them decreases heat flow (higher R-value/lower U-value). Double-paned windows usually have U-values of 0.55 or lower. Triple-paned windows have U-values of 0.333 or lower.
The width of the air spaces between the panes has varied (usually from 1/8" to 1/2"), with resulting higher or lower R-values. For example, >5/8 inch (or 1.6 centimeters) air space is too wide and <1/2 inch (or 1.3 centimeters) is too narrow, each resulting in lower R-values. Multi-pane windows are more expensive than single-pane windows and limit framing options because of their increased weight. Savings are most often made up with lower energy bills.
Advanced, multi-pane windows are now manufactured with inert gas filling in the spaces between the panes instead of air. These gases allow less heat transfer than air, increasing the window’s insulating capabilities. Although air is cheaper, it is much less efficient then an inert gas fill. Argon and krypton are the usual gas choices. Gas filled double-paned windows can achieve U-values of 0.333 or lower.
Argon gas has a lower conductivity than air, thus reducing conduction heat loss. It is also heavier than air, curtailing movement between the glazings and thereby reducing convection losses. Argon is the more cost-effective gas. Many manufacturers offer it either as a standard feature or as an inexpensive upgrade.
Krypton gives better energy performance than argon. It also requires less spacing between panes, about 1/3 inch (or 8 mm), and therefore less gas. A narrower space allows multiple-pane systems with less chance of stress breakage. Krypton has a higher R-value, but it is also more expensive than argon.
Replacing the traditional primary window glazing material, clear glass, are technologically advanced glazing systems developed specifically to help control heat loss and condensation. Glazing basically takes solar radiation from the sun and directs it in one of three different ways. Some rays are reflected off the window (low-e and reflective glass), some go directly through it (traditional single pane and smart windows), and some are absorbed by the glazing (heat absorbing, gas filled) which warms up the window and keeps it close to room temperature.