Insulators are materials that have structural properties exactly opposite of conductors. These materials are made up of atoms whose electrons are not easily "freed". These electrons are said to be tightly bound to the nucleus, and are very stable.
Insulators are used to prevent the flow of electrical current. The rubberized power cord and plastic coverings on appliances are typical examples of insulators. Glass, rubber, porcelain, and most plastics are good insulators.