The electrons in the inner shells are very tightly bound to the nucleus but those on the outer shell are not so tightly bound. The electrons on the outermost shell are called valence electrons.
fig. 1.3 Covolent bonding
Fig. 1.4 Movement of charge carriers
How can a hole move? It is actually the electrons that move - consider a dentist's waiting room (Fig. 1.5). You can see that it is an effective movement of holes caused by the movement of electrons.