Substances that allow an electric current to flow through them are called conductors; those which do nor are called insulators. Metals are conductors. In a metal structure, the metal atoms exist as positive ions surrounded by an electron cloud. If a potential difference is applied to the metal, the electrons in this cloud are able to move and a current flows. When the electron are moving the metal structure, they bump in to the metal ions and this causes resistance to the electron flow or current. In different conductors the ease of flow of the electrons is different and so the conductors have different resistances. For instance, copper is a better conductor than iron. Here is the list of materials on the basis of their nature.