Rust on metal is caused by surface metal atoms and molecules forming metal oxides in the presence of oxygen and water. Iron is the most common material that rusts, forming iron oxides, but most other metals can rust as well.
Rust was originally a synonym for iron oxides, but is now the general term for metal corrosion. It is a flaky and fragile substance, which is undesirable for most applications that metals are used for. The rate at which a metal rusts depends on its surface area and environment. Moist environments cause significant rust, while salt water accelerates rusting even further.