Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice. In an atom, the electrons in the outer most orbit are called the valence electrons. If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.
If the energy levels are graphically represented, we will get a band diagram.
In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band.
Valence Band
The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom.
Conduction Band
The conduction band is the range of energy levels all electrons which are involved in conduction.
In conductors, the valence and the conduction bands overlap. In Insulators, the valence and conduction bands are far apart.
In semiconductors, the distance between the valance and the conduction bands are small. When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band.
The difference between the valence and the conduction band is called the energy gap.