Rusting is the process in which iron turns into iron oxide. It happens when iron comes into contact with water and oxygen. The process is a type of corrosion that occurs easily under natural conditions.
Corrosion is an electrochemical process that requires a cathode, an anode and an electrolyte. An anode is a material that readily releases electrons, while a cathode is a material that readily accepts electrons. An electrolyte is a liquid that allows electrons to move about freely. Rust is such a common occurrence that little pure iron is found in the world. Under most circumstances, it transitions into iron oxide.