Materials are often classified into three distinct categories: insulators, semiconductors, and metals.

The three materials are distinguished from one another based on their band gap, or by their resistivities (conductivities).  Roughly speaking, metals are materials with bulk resistivity less than 10^{-3} \Omega \cdot cm.

Energy band theory can explain these differences in resistivity in terms of differences in band gap.  Every material has a band structure associated with it that results from electrons interacting with one another.  Those band that are filled are called valence bands, and those that are empty are referred to as conduction bands.  The difference between the bottom of the conduction band and the top of the valence band is called the band gap.

If the band gap is above 3 eV, then the material is generally considered an insulator.  If the material has a band gap of less than 3 eV, then it is generally a semiconductor.  Metals do not have a band gap, and instead have partially filled bands.