A very rarely used material. Less durable than materials such as steel, rust is created by the corrosion (Oxidization) of ferrous materials such as Steel and Iron.
See Rust items for a (short) list.
In PnP, Rust Monsters have the ability to corrode metallic items they touch. It's similar in DDO, metallic weapons that strike Rust Monsters take highly increased weapon damage and often break after just a few hits. Additionally, Warforged characters suffer additional penalties, see details.