I realize this post is a bit old but the earlier replies are incorrect. Low-e windows come in a wide variety of types with a range of performance options. Emissivity (the E in Low-E) is the measure of how efficient the item in question (windows) is at retaining thermal energy (heat). A lower emissivity means your window will retain more of the heat you have inside of your home by not letting it radiate through the window. This is completely separate from air sealing performance and the SHGC (Solar Heat Gain Coefficient - the measure of how much solar heat enters the window on sunny days).

For cold climates with a passive solar design, a High SHGC/Low-E window gives you passive heat gain during the day and retains more of the heat during the night. Make sure you talk to an educated professional and know all of the performance of your new windows before committing to a purchase. We have seen hundreds of clients over the years overspend for windows that were not the right choice for their needs.

You can also achieve similar performance with the addition of Low-E window films... Best of Luck!