In short, rust is a name for iron oxide, formed when iron is oxidized in the presence of oxygen and water. A simple iron oxide, not formed in a reaction with water, is iron (III) hydroxide.
4Fe + 3O2 = 2Fe2O3
Because rust is flaky and not as strong as iron, preventative measures are often taken to prevent the weakening of metal. The most common method of protecting from rust is galvanization, protection from rust through the application of a protective layer of metals such as zinc. Galvanization usually occurs by either hot-dip galvanization or electroplating.