In a single isolated atom, the electrons in each orbit have definite energy associated with it. But in the case of solids all the atoms are close to each other, so the energy levels of outermost orbit electrons are affected by the neighbouring atoms.
When two single or isolated atoms are brought close to each other than the outermost orbit. The electrons in the outermost orbit of one atom experience an attractive force from the nearest or neighbouring atomic nucleus. Due to this, the energies of the electrons will not be at the same level, the energy levels of electrons are changed to a value which is higher or lower than that of the original energy level of the electron.
The electrons in the same orbit exhibit different energy levels.The grouping of this different energy levels is called energy band.
However, the energy levels of inner orbit electrons are not much affected by the presence of neighbouring atoms.
Insulator is material, in which there is a large energy gap between the conduction band and the valence band, A large amount of energy is required to shift the electron from valence band to conduction band.
Semiconductor is the material, in which there is a less energy gap required with respect to the insulator, When some amount of energy flow in the semiconductor, then it allows to flow the electron from valence band to conduction band.
Conductor is the material in which, there is no energy gap between the conduction band and valence band, so no extra energy is needed to flow the electron from valence band to conduction band.