All kinds of metals will react with oxygen and form an oxidation film. Compared with other metals, carbon steel may produce iron oxides, which will continue the oxidation, intensify corrosion, and create small holes. Although processes like painting or electroplating can protect it from corrosion, carbon steel still has the risk of rust if scratched.
Stainless steel can form a thin passive film of chromium oxide when exposed to oxygen, so as to prevent the surface from further oxidation. Covered with the film, stainless steel looks smooth and lustrous. Besides that, scratched area may produce a passivation layer again, which makes it reliable and durable.