R-Value is a measurement tool for thermal resistance, or the capability of a material to resist in front of the movement of heat.
U-Value is the thermal resistance or the amount of heat which is lost through the structure of a material. The U-Value is measured on a rate which heat moves through one square meter of a structure.
Low E glass is a type of glass with low-emissivity. As a example, a double-pane window with a low E coverage has a U-Value of about 0.36 which in another word, with 35% less heat gain or loss in compare to standard double-pane windows.