The red rust you see is actually called iron oxide. This is a very common material as iron (and steel) combines very easily with oxygen, making iron oxide. This is why most iron and steel you see will actually have a coating on them, preventing the air (oxygen) from getting to it. Water (or other liquids known as electrolytes) helps to provide oxygen to the metal, and the metal corrodes, either being swept away or converted into rust.
Cars rust where their outside coating has been removed – either through scratches or from wearing away. The rain water acts the the electrolyte, and rust is formed