Put simply, it is a type of steel. More specifically it is a high-strength, low-alloy structural steel created by United States steel in 1930 to resist abrasion and corrosion and used largely in the shipping industry to make shipping containers.
Developed by mixing a particular combination of steel and alloying elements, COR-TEN steel (also written as corten and cor-ten) is known as 'weathering steel' due to the protective layer of rust that weathers - and protects - the material. This protective rust ‘patina’ on the surface prevents further corrosion from penetrating deeper into the metal.
Note: The most notable Cor-Ten structure in the UK is the Angle of the North!