Glass, paper, Teflon and rubber are good insulators of electricity. An insulator is a substance that allows very little electric current to flow through it. There is no perfect insulator, but materials that offer a high degree of resistance to the flow of electrons are good insulators.
All substances offer resistance to the flow of electric current through them. This is called the resistivity of the material. Different materials have varying resistivity levels. The higher the resistivity of the material, the less electric current passes through it.
Rubber has high resistivity of 1-100 x 1013ohm-meter, making it a good insulator. Copper has a low resistivity of 1.59 x 10-8 ohm-meter, which makes copper a good conductor of electricity, or a poor insulator.