Houzz have this US biased article on Energy-Efficient Windows: Understand the Parts
Window Frames
All have their advantages and disadvantages, but the U.S. Department of Energy (DOE) recommends vinyl, wood, fiberglass or composite. (Aluminum is easily heated, so it isn’t a great insulator.) Wood has efficiency problems due to expansion and contraction, so consider aluminum- or vinyl-clad windows. But if you choose vinyl, consider an insulated version, depending on your climate and house requirements.
Window elements from http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13370
Window Glazing
Different gas fills impact cost and efficiency.
The most common gasses used are argon, which is relatively less expensive, and krypton, which is more expensive but has a better ability at decreasing a window’s U-factor.
Tints and inner glass layers or spectrally sensitive coatings can impact the U-Value.
Low-E coatings. These coatings increase a window’s cost by 10 to 15 percent, the DOE says, but they can reduce energy loss by about 30 to 50 percent.
For keeping heat inside the house in the winter, the coating should be on the inside of the glass.