Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Given sufficient time, any iron mass, in the presence of water and oxygen, could eventually convert entirely to rust. Rusting is the common term for corrosion of elemental iron and its alloys such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called "rust".
In the Oz universe, rust is the Achilles heel of the Tin Woodman, who in The Wonderful Wizard of Oz (1900) was paralyzed for a year by rust after being caught by rain while chopping wood. Dorothy and the Scarecrow found him, and lubricated his joints with his oilcan to restore his mobility. Rust also plagues him when he sheds tears, caused on one occasion by his grief when Dorothy is poisoned by the accursed poppies. Similar mishaps bedevil him throughout most of the Oz books in which he appears.
In the real world, tin is not one of the metals that are subject to rust. This discrepancy bothered Alexander Volkov, who reimagined Oz for a Russian/Soviet audience. In all of his Magic Land books, the character is called the Iron Woodman for better verisimilitude.