ISO 216 is the international standard by which paper sizes are determined. The system establishes three set of paper sizes, known as the "A", "B" and "C" series.
A3, A4, A5 and etc is commonly used paper size in the world. Here are the chart of sizes available for A series.
Using same aspect ratio between height and widht as the A series, B series sheets appear to be same shape as A sheets. However the sizes are slightly different.