Stainless steel is a special form of steel that is made of chromium added to steel. Regular steel is an alloy that is made of both iron and carbon. There are different types of stainless steel depending on how much chromium is added and if there are any other elements added in small amounts like nickel.
As the name suggests, stainless steel is highly resistant to stains, in particular rust. The chromium added to the steel creates a thin barrier composed of chromium oxides, which effectively limits the amount of oxygen that can penetrate the material and cause rust to form.