An electrical insulator is a substance that is very poor conductor of electricity. A conductor has many free electrons (roughly one per atom), which are free to move throughout the material, while in an insulator essentially all the electrons are bound to the atoms. A material that is a poor conductor of heat generally is also a poor conductor of electricity since both phenomena largely depend on the transport of energy by electrons.
The distinction between conductors and insulators, although not absolute is dramatic. It may be expressed in term of the resistivity, which is resistance in ohms or a piece of material 1 cm long, with a cross sectional area of 1 cm. the difference between the resistivity of copper, an excellent conductor, and that of glass, a typical insulator, is more than 1020, 100 billion billion.
Immediately between conductors and insulators are Semiconductors, which can have resistivities of the order 1 ohm-cm. These include substances like germanium and silicon. Impurities strongly alter the conducting properties of such materials. For example the addition of boron to pure silicon in the ratio of 1:100,000 increase the conductivity a thousand fold room temperature.
The principle of materials that is used for insulator is bad conductor. Materials use for insulators like plastic, rubber, glass, ceramic and others. Every materials may use in different condition and purpose. Some insulator can use for a body or box of electronics unit, others may must resist to chemicals condition like on car battery. Other purpose need insulator that can keep has as insulator and not damage on high temperature conditions or on cold conditions.