a) metals, (b) insulators and (c) semiconductors
Two distinguishing features :
(i) In conductors, the valence band and conduction band tend to overlap (or nearly overlap), while in insulators they are seperated by a large energy gap and in semiconductors they are separated by a small energy gap.
(ii) The conduction band of a conductor, has a large number of electrons available for electrical conduction.
However, the conduction band of insulators is almost empty while that of the semi-conductor has only a (very) small number of such electrons available for electrical conduction