“How do we classify materials that do not let charge flow freely?”
For most of our study of electromagnetism, we have been studying materials that allow electrons to flow freely. But out of curiosity, are there materials that do not allow such a free movement of electrons? If these materials do in fact exist, then trying to induce an electric current to flow through them would be futile, as the electrons would not even budge. Believe it or not, these objects do in fact exist, and scientists and engineers classify these materials as insulators. Insulators have many pragmatic purposes, such as dielectrics and high voltage systems.