Rust is the normal electrochemical process of metals returning to their natural unrefined state. It occurs when iron and oxygen combine. Moisture, salt and high temperature increase the rate of chemical reaction.
Motor Vehicle Sheet Metal (Steel) is made from approximately 99.7% Iron and 0.3% Carbon. Iron comes from Iron Ore. Iron is highly reactive under the action of Oxygen and water.
Very simply stated, iron (+) and oxygen (-) have opposite charges so they seek out each other to combine (opposite charges attract and like charges repel). When they combine, the iron loses electrons to oxygen atoms. This is called oxidation and when it happens it produces a chemical reaction forming Iron Oxide Fe2O3 commonly known as rust.