Rusting is the common name for the slow oxidation of iron. More precisely, rust is a mixture of hydrated iron oxides and iron oxyhydroxides that forms when iron or steel is exposed to oxygen and moisture.
The process is electrochemical. Iron atoms lose electrons, while oxygen gains electrons in the presence of water. The resulting corrosion products are usually reddish-brown, flaky, and weakly attached to the surface. Because rust flakes away, it exposes fresh iron and allows corrosion to continue deeper into the object.
Salt water and road salt speed rusting because dissolved ions make the water a better electrolyte. Rust also occupies more volume than the original iron, so expanding corrosion can crack concrete around reinforcing steel, weaken bridges, and jam moving metal parts.
Other metals oxidize too, but their corrosion products are not normally called rust. Aluminium forms a thin, tightly adherent oxide layer that helps protect it, while copper develops a distinctive green patina. The word rust is used especially for corrosion involving elemental iron and iron alloys such as steel.