When I first looked at the NFRC sticker on a window I thought it looked like the nutritional label that you can find on packaged food items. Basically it performs the same function. The food label tells you how many calories are in an item and breaks it down by carbohydrate, protein, and fat so that you can make an informed decision on what foods you want to incorporate into your diet.
The “Nutritional” labels on a window tell you what properties that particular window has so that you can make informed decisions on what products you want to incorporate into your building. The labels contain information from the National Fenestration Rating Council (NFRC). These are the folks that certify that the windows you purchase actually perform to the level that the manufacturers say. It is important to note that these ratings are for the entire window assembly. (By the way, according to Wikipedia, fenestration when it relates to construction is a word that means a planned opening in a building like a door, window, or skylight.)
So what kinds of things does the NFRC label tell us? Currently there are a few required pieces of information, like the U-Factor (how much heat it conducts, which is opposite of the R factor that you are used to seeing associated with insulation which shows how much heat it resists. For U values, the lower the number the better), Solar Heat Gain Coefficient (how well the window performs at keeping heat from the sun from entering the building, the lower the number the less heat is allowed in), and Visible Transmittance (how much light is allowed through the window – the higher the number the more light gets in). A few other bits of info like the amount of Air leakage and Condensation Resistance are optional.
But wait! There’s more! “The Council has also developed rating and labeling procedures for window film attachments, internal or between the glass blinds and shading devices, and dynamic glazing products that change tint in response to temperature, sunlight, or electric charge. The Certified Products Directory is expanding to include these ratings as they become available. NFRC is currently considering such energy ratings as ultraviolet light/fade protection, and overall comfort.” 
So how do I know which window to pick for my Passive House? Because there are so many choices of materials, coatings, films, gases, glasses, and frames (don’t forget the spacers!) choosing the windows requires some homework. Fortunately there are a variety of tools that Passive House consultants and window manufacturers have that can make it easier. The fine folks at Berkeley National Labs have developed some modeling programs (Windows, & THERM) for designing windows and testing their performance before you even have cut the glass. The Passive House Planning Package (PHPP) allows your consultant to model your construction with any variety of window products that have any number of different qualities so that you can try out a variety of sizes, configurations, materials, and pricing structures before the building is even begun. They can even use the Berkeley Labs THERM software to test the thermal performance of the rest of the structure too!
The idea that you are designing a building to function as a complete unit and not just choosing a window to complete a design aesthetic means there is more to choosing a window than you would think, but because Passive Houses ARE designed with planned performance, you can still plan on having the design aesthetic you desire as long as it works within the PHPP software. Obviously a greenhouse built in Antartica would never be able to perform as a Passive House, but you could actually model how it would perform with the software that is available so that you would know before construction what to expect.
So… how nutritious are YOUR windows?
For additional information on window performance and certification, be sure to check out the following links:
National Fenestration Rating Council - http://nfrc.org/default.aspx
Berkeley National Labs - http://windows.lbl.gov/