There are a lot of acronyms and ratings in the window industry that explain how one window is different from another. Here are a few key items to keep in mind when choosing a window:
Visible Light Transmittance (often referred to as VLT). This is the amount of daylight that will be allowed into your home with your new product.
U-value (the inverse of R-value) is used to measure the efficiency of your new windows. Make sure to get an accurate figure based on the entire window unit and not on the center of glass or the frame alone.
Solar Heat Gain Coefficient (also called SHGC) is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a windows solar heat gain coefficient, the less solar heat it transmits into your home. A lower SHGC relates to lower cooling costs for your home in summer.
Aesthetic appeal. Since you will have to look at your new windows for a long time to come, it is important that they are visually appealing on the interior and exterior of your home.