In the float glass process, a continuous strip of molten glass, heated to more than 1000 degrees Centigrade is poured from a furnace on to a large shallow bath of molten metal, usually tin. The glass floats and coos on the tin and spreads out to form a flat surface. The speed at which the controlling glass ribbon is drawn determines the thickness of the glass.
You can learn more about the process in the video below.
Here are some examples of the float glass process in action:
From PPG Industries