Dual-pane windows are energy efficient windows that consist of two glass panels set in a frame, separated by a small space (1/2 inch to 3/4 inch wide). The space between the layers of glass are filled with nontoxic, inert gasses (such as argon) which add to the window’s insulating properties. The glass itself is often coated to reflect heat as well.

To learn more, go to:
http://www.consumerenergycenter.org/home/windows/todays_windows.html

http://www.energystar.gov/index.cfm?c=windows_doors.pr_windows