Oxidation is the reaction that occurs by adding oxygen to other materials, often in combination with water. Oxidation is commonly called rust, but this term technically only applies to iron. Oxidation can occur in a variety of substances besides metals, including food and other materials.
When oxygen in the air reacts with the molecules that make up many substances, the result is often destructive. Rust forms on a car when the outer layer of wax degrades, and the metal is exposed to oxygen and moisture. Rust is not only unsightly, but it is also a brittle material that inhibits the proper functioning of metal components.