This is one I hear often from people considering replacing old windows with new double pane vinyl windows. Low-E glass is glass that has been treated to be more energy efficient. It helps keep the summer suns energy from heating your home, and helps prevent heat loss during winter months.
Other insulating options are available too such as argon gas filled double pane windows. Again, an options, this gas helps prevent the transference of outside temperature differences to the inside of your home. You can check out these resources for a more detailed look at Low-E glass and argon options for your home.
