A useful way to visualize the difference betweeen conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. When two or more atoms approach one another, instead of having discrete energy states, the available energy states split into bands of closely separated levels. Each band will in turn be separated by energy gaps between them.
The overlapping electron wave functions of the high energy levels (the conduction bands) extend across the whole solid. The electrons in these levels are ‘free’. The electrons in the lower levels (the valence bands) are bound to their respective atoms. Crucial to the conduction process is whether or not there are mobile free electrons in the conduction band.
Another important parameter in the band theory is the Fermi level, the maximum available energy level at low temperatures. The position of the Fermi level with relation to the conduction band is a crucial factor in determining electrical properties.