Insulators are materials that do not allow charge to flow through them. They block the flow of electrons. All the materials that can be charged by rubbing are insulators. This is because if you tried to charge a conductor by rubbing it, the charge would immediately run away through you to the earth. However, if you charge up an insulator, the charge just stays there. It would be difficult to get it off. Insulators are the opposite of conductors. Also see semi-conductor.