Conduction is the passage of energy, particularly electrical or heat, through matter. See also the entry for heat conduction.
Metals
A metal can be regarded as a lattice of positively charged metal ions surrounded by a 'sea' of mobile electrons that are not bound to any particular metal nucleus.
These electrons are described as occupying the metal's conduction band.
If an electric potential from a battery is applied to a metal, forming an electric circuit, it will cause a net drift of electrons to flow around the circuit. The higher the potential, the greater the flow of electrons.
Although individual electrons move only very slowly though the metal - the net electron drift speed is a few mm per hour - the electric signal travels at the speed of light in the form of an electromagnetic wave. Electrons start drifting to form an electric current as soon as the electromagnetic wave reaches them. As a result of this, electric current appears to flow at the speed of light, even though the electrons forming the current actually move through metals slowly.
The electrical resistance of metals falls with temperature. In some metals, at temperatures close to absolute zero, a critical temperature exists below which the metal becomes superconductive: resistance falls to zero. A current flowing in a superconductive metal will flow for as long as the metal remains at a low enough temperature to maintain its superconductive state.
Low temperature superconductivity in metals results from the interaction of electrons with phonons.
Semiconductors
Conduction in semiconductors is also by electrons.
However, semiconductors do not have a convenient 'sea' of free electrons to carry electric current.
By applying an appropriate electric potential to a semiconductor, some of its electrons can be given enough energy to move freely though the material.
These electrons are said to have been promoted into the semiconductor's conduction band.
Electrical conduction through semiconductors is poorer than through metals, because fewer electrons are available as charge carriers.
The resistance of metals increases with temperature, because the flow of electrons is impeded by lattice vibrations. In semiconductors, however, at higher temperatures, more electrons get sufficient energy to enter the conduction band; therefore semiconductor resistance falls when the temperature rises.
Dissolved and Molten Salts
If an electric potential from a battery is applied to an ionic salt dissolved in an appropriate solvent, such as water, or to a molten ionic salt, an electric current may flow.
In this case, the charge carriers are mobile, electrically charged ions.
Solid salts do not conduct electricity because they lack mobile charge carriers.