Free Charges and Bound Charges Inside a Conductor

Going one level deeper than the conductor/insulator distinction itself: in a metallic conductor, the electrons occupying the OUTERMOST electron shells of each atom are only loosely bound to their parent nucleus, and readily detach to roam freely throughout the whole body of the metal. Once an external field is applied, these mobile electrons drift steadily in the direction OPPOSITE to the field (since electrons carry negative charge) -- this population of readily mobile electrons is collectively called the FREE CHARGE of the conductor.

The nucleus of each atom -- carrying the positively charged protons -- together with the INNER-shell electrons that remain tightly bound to it, stays essentially fixed in position at all times and never migrates through the material; this immobile population is called BOUND CHARGE. …