A body is said to possess electrification when it attracts other bodies, the attraction being different from the gravitational attraction. Electrification is not a fundamental property of matter because it remains un-electrified under ordinary conditions and it becomes electrified only after the electrification has been produced by certain causes.
All substances may broadly be divided into groups:
1. Conductors
2. Non-conductors.

Conductors:
In case of a conductor, if one point of the body of the substance is by any means electrified, the electrification immediately spreads all over the body or in other words, conductors are the substances which are used to produce easy and smooth flow of electricity. The best conductors are the metals and solutions of most salts in water. Wires made of copper, aluminium, etc. are widely used as conductors.

Non-conductors:
In case of a non-conductor, the electrification produced does not spread over the entire body, but it remains in the neighbourhood of the point where the electrification took place. The non-conductors are also known as insulators. There is no substance which can be considered as a perfect insulator. But glass (especially when it has been boiled in water and is been kept in a dry atmosphere), paraffin, sulphur, plastics, rubber, asbestos and fused quartz are sufficiently good insulators for all practical purposes. Petroleum, oil, etc. are the important liquid insulators.

Electric insulators may be in the form of solids, liquids or gases. But solid and liquid insulators are important from the commercial point of view. When a gas is in the normal condition, it conducts electricity to a very small extent. But it can be made to conduct electricity freely, when it has its temperature raised sufficiently and when it has been traversed by rays given out by certain substances such as uranium, radium, thorium, etc.

The term dielectric is used to mean a substance capable of supporting an electric stress and its strength or power to resist an electric stress is known as dielectric strength. The properties of a good electric insulator can be summarized as follows:
1. Desired chemical stability,
2. High dielectric strength,
3. High electrical resistance,
4. High moisture resistance,
5. High thermal resistance,
6. Low dielectric constant,
7. Low power factor,
8. Suitable mechanical and physical properties for efficient working, etc.

It may however be noted that there is no hard and fast line of demarcation between the two groups mentioned above. Certain substances such as dry wood and paper have intermediate properties and hence, they are sometimes referred as semi-conductors.