Removing rust that is on the surface of the vehicle is as easy as sanding down the rusty area with sandpaper, applying primer and repainting the surface. Deeper rust may require using a sanding wheel to remove the rust or replacing the affected body part. If the body part in question has been completely eaten through to the point where the rust has eaten holes in the metal, it will need to be replaced.
Rust is the breakdown of iron-based metal in a process called oxidation. During oxidation, metal combines with oxygen in the air and creates iron oxide, otherwise known as rust. Given enough time, iron oxide can completely eat through iron-based metals.
Thanks to advancements made by auto manufacturers, surface rust is now the most common type of rust on automobiles. Today's manufacturers use magnesium and aluminum in car construction, both of which corrode at a much slower rate than what was used back in the 1970's. Cars and trucks whose bodies are made of steel typically get a special coat to protect against rust. Owners of older models may still have to deal with rust.
Deeper rust forms over time and over the course of an automobile's lifetime, and can eat holes in the frame and other parts. In this case, metal that has "rusted out" should be replaced, especially if it compromises the safety of the vehicle.