Generally there are four kinds of materials:

1. Conductors: Materials which allow electrons to flow through them, like metals. In conductors, the outermost electrons in each atom are so loosely held that they are able to move freely between atoms. These free electrons also make metals very good conductors of heat.

2. Insulators: Materials which do not easily allow electrons to flow through them, like plastic. These materials do not conduct charges, because their electrons are all tightly held to atoms and are not normally free to move. The insulators are relatively easy to charge by rubbing, because any electrons which are transferred tend to stay where they are.

3. Semiconductors: Materials which are partially conductors, and partially insulators, but become very good conductors when tiny impurities are added to them.

4. Superconductors: Materials which acquire zero electric resistance (infinitely conductive), the electron flowing indefinitely without losing energy when passing through a superconductor.