ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to help you save money and protect the environment though energy efficient products and practices. The NFRC (National Fenestration Rating Council) is a third-party, non-profit organization that independently tests and certifies all ENERGY STAR qualified windows so you can compare the energy and performance features.
The NFRC label can be found on all ENERGY STAR qualified windows, doors, and skylights and provides performance ratings in five categories:

U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h•ft2•°F. The lower the U-factor, the better the window insulates.

Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.22 to 0.80. The lower the SHGC, the less solar heat the window transmits.

Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.

Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf•m/ft2.

Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows.