NFRC (National Fenestration Rating Council) label is used to help the consumer make an educated purchase by allowing comparison of the energy efficiency between windows. You will find four ratings; U-Factor, Visible Transmittance, Solar Heat Gain Coefficient and Air Leakage.
The U-Factor lists the amount of heat loss through the window, which is important when you are trying to heat your home in the winter. The lower the number the better.
Visible Transmittance as you may guess by the name measures that amount of natural light that the window allows into the home. With this measurement the larger the number the better.
Solar Heat Gain Coefficient is the measurement of the amount of heat gain through the window, which is good to know when you are trying to cool your home in the summer. The lower the number the better.
Air Leakage calculates how much air can pass through a window creating drafts. You want to look for lower numbers here as well.
To learn more about efficiency check out their website at www.nfrc.org.