Busbar is used to provide a continuous electrical connection around the perimeter of the glass between the connector (inputting the electrical supply) and the heating element (tungsten wire or metallic film).
The busbar is always based around an ultra-thin, very soft copper with a solder coating. The thin property of the foil allows it to be applied within the two layers of glass (actually directly onto the PVB) without creating any significant dimensional increase that can leave air gaps in the screen. The soft property allows the busbar to be easily shaped when it is applied around the commonly curved glass perimeter.
PVB (Polyvinyl butyrate) is a clear soft plastic interlayer which is inserted between two layers of glass, and protects the driver from the risk of glass fragments should the windscreen break.