Amateur Radio Manual/Conductors and Insulators
Certain elements, like copper in our discussion on atoms, have their valence electrons held weakly in their outer orbit. By supplying energy to the material (one way would be to heat it), we can make the electrons move randomly from one atom to the next. We could also attach a negative source of energy that repels the electrons and attach at the other end a positive source that would attract the electrons. This would force the electrons to move from the negative end to the positive end. We call that net movement of electrons 'electric current'. Substances that have this ability are known as conductors, and most metals are conductors of electrons.
Insulators, on the other hand, have their valence electrons bound tightly to the nucleus of the atom. They will not, under normal circumstances, allow the electrons to move easily by supplying energy to the material. However, note that by supplying enough energy (like a lightning strike) we can force even tightly held electrons to move.
Common insulators are porcelain, rubber, glass, and plastic. Be cautious with plastics however, some of them can be very good conductors and it is not easy to tell one from another.