Conductors and Insulators

Materials are classified according their conductivity, or ability to conduct electricity. Metal atoms usually have their outermost (i.e. valence) electron(s) attached to their parent nucleus rather weakly. These electrons are easily detached and can flow freely throughout the metal. They are called free electrons. The high mobility of the free electrons, and their large number, make metals good conductors of electricity (and also heat).

Substances like rubber, glass, wood, etc., on the other hand, have all their electrons tightly bound to their parent atoms. They have very few free electrons and are therefore extremely poor conductors.

Conductors => Electric charge can flow very easily, that is with very little resistance, due to the large number of mobile free electrons.
Example : Most metals such as copper, aluminum, iron, gold, ... are good conductors.
Insulators => valence electrons are tightly bound - few free electrons, thus no conduction of charge.
Examples: Glass, rubber, plastics, ceramics, ...
Semiconductors are somewhere in between these two extremes. They are rather more like insulators, in which few electrons are available for conduction. The conductivity of semiconductors can be controlled by small amounts of 'doping', the addition of very small, precisely measured amounts of certain specially chosen materials.
Examples : silicon, germanium, gallium arsenide, .... . (Practically all electronics, including computers, are largely based on the use of semiconductors.)

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Although dry air is an insulator, humid air can conduct electric charge to some extent. On a humid day, a charged conductor gets slowly discharged via the leakage of charge through the air, even if it is insulated.

If the charge on a conductor is very large, then discharge through the air can happen via a spark or lightning