E-coatings or Low-emissivity coatings are thin coatings that are put on top of traditional glass windows that prevent or limit certain wavelengths of light from passing through the pane while still allowing the passage of visible light. These coatings reflect ultraviolet or infrared light to increase the insulating performance of the glass leading to increased energy efficiency while still maintaining the appeal of a clear window.
It is desirable to limit the entrance of ultraviolet light  into the home as they cause interior materials such as fabrics and wall coverings to fade. Infrared light transmits heat into the home. Infrared from the sun is known as short-wave infrared, whereas infrared from other warm objects is known as long-wave infrared. Preventing the passage of this type of light also prevents the movement of heat from where it is needed, see figure 1.
With traditional clear glass windows, significant amounts of solar radiation pass through the window. This heats objects inside the home, which is undesirable if the temperature is above what is comfortable. In addition, warm objects inside of the home can heat the glass of the window up, and the window then re-radiates this heat into the outside. This leeches warmth from the inside of the home which may not be desired. Ideally, a window is able to prevent heating from the outside during warm weather months while keeping the temperature inside during the cool months. The ability of the glass or any other material to radiate and re-radiate energy is known as its emissivity, and thus highly reflective materials generally have a low emissivity. The ability to lower the emissivity of the glass surfaces in a window improves its insulating properties.
Low-e glass has a thin, transparent coating, generally comprised of a thin layer of silver or other low emissivity material, that reflects infrared energy. Simply put it reflects heat (like a heat mirror) and keeps it inside (see figure 1). This prevents heat transfer between the inside and outside environments. Interior heat energy is reflected off the low-e coating, returning the heat back to the inside and reducing radiant heat loss through the window. By placing low-e glass into a window, the individual temperatures of the panes change. More heat is retained by the window and the outer pane is heated less by escaping energy, causing the internal pane to become warmer. This then prevents cold drafts from convection near the windows as well as reducing the condensation on the glass. This is how the glass keeps a room warm during the colder months.
Low-e coatings come in two types, passive low-e coatings and solar low-e coatings. These are manufactured in two different ways. There are several different portions of a window that these coatings can be applied to. These correspond to four "faces" of the panes in a double glazed window. For each panel of glass, the low-e coating can be applied to the inner or outer facing side of the pane.
Passive low-e coatings are manufactured using a pyrolytic process. In this process the e-coating is applied to the glass while the glass itself is being produced. This causes the coating to fuse to the hot glass, creating a strong bond between the coating and the glass. The glass is then cut into sheets to make windows. For these types of coats, it is generally a layer of low-emissivity indium tin oxide that is deposited on the window.
Passive low-e coatings are generally used in cold climates as they still allow some of the heat energy from the sun to enter the home and help heat the building during the winter. However they still reflect heat energy from the inside that allows heat energy to stay in the home. Due to the fact that these glazings let in more solar energy, they have a do not have a good R-value (or U-value), but they are better for houses that rely on passive solar heating because more light and heat can enter the home.
Solar control low-e coatings are manufactured using a process known as the MSVD or sputtered process. In this process, the coating is applied to pre-cut glass in a vacuum chamber. The coating is not as firmly attached as those applied in a pyrolytic process, so a seal and/or laminate must be applied to keep the bond strong. This type of coating has a lower emissivity and thus lets less solar energy through the window. For these types of coats, it is generally a thin layer of silver that is deposited on the window.
Solar control low-e coatings are generally used in hotter climates where keeping external heat from entering a building is more important.