Stainless steel remains "stainless" if it contains at least 10.5 % chromium. It is this element that reacts with the oxygen in the air to form a surface layer that is invisible but strong enough to prevent further oxygen from "staining" (rusting) the surface.

The first number is the amount of chromium that is contained in the stainless, i.e., 18 is 18% chromium. The second number is the amount of nickel, i.e., 8 stands for 8% nickel. So 18/8 means that this stainless steel contains 18% chromium and 8% nickel. 18/10 is 18% chromium and 10% nickel. The higher the numbers the more corrosion resistant the material.

The 300 series (which contains nickel) is NOT magnetic. The 400 series (which just contains chromium and no nickel) ARE magnetic.