Definition: In the industry, panes of glass or the act of installing panes of glass in a window is called glazing.
Layers of Glass (or Glazing) in Replacement Windows
If you are replacing your windows, you should not just get single-pane glass windows. It has very little insulating value compared to a double-pane glass window and additional the cost of double-pane window is easily overcome with the energy savings in your home. Not to mention, you will also have acoustic insulation from the double-pane window, so your house will be much quieter.
It is possible to get triple-pane windows, but that is only necessary if you need the additional sounds insulation. If you live near a busy street, triple-pane windows are your best options. Otherwise, double pain should be enough acoustic and thermal insulation.
Types of Glass (or Glazing) in Replacement Windows
Window panes in homes most commonly use plain clear glazing. With the advancement of technology it is now possible to have different glazing options that offer heat loss and condensation improvements.
The most common is low emmisivity (low-e) glass. Low-e glass has a special coating on the surface to reduce the heat transfer through the window. They allow all the light to pass through, but reflect 35-65% of the heat that would normally pass though.
Heat absorbing glass has a coating that absorbs up to 1/3 of the incoming solar heat. It will reduce the amount of light that enters the house. Reflective glass has a coating that relects much of the heat energy and will also reduce the amount of light that enters the house. Both these glazing options are good in environments where it is normally hot and there is a lot of light. It would not be recommended for more northern climates, because you would reduce the light you get in the winter.