Corrosion is the gradual destruction of a metal by reaction with substances in its surroundings, such as oxygen and water. When a metal corrodes it is oxidised, which means it gains oxygen to form a metal oxide.
The corrosion of iron and steel has its own name: rusting. Iron reacts with oxygen and water to form hydrated iron(III) oxide, a flaky orange-brown solid called rust. Rust is weak and falls away, which exposes fresh iron underneath, so rusting keeps weakening the object until it fails. Only iron rusts, but other metals corrode too. Copper roofs slowly turn green as a layer forms on the surface.
Some metals protect themselves. Aluminium reacts with oxygen to form a thin layer of aluminium oxide that sticks firmly to the surface, so the metal underneath does not corrode any further. Iron is not protected in this way, which is why corrosion of bridges, ships and pipes costs so much to repair.