Band theory is a model that explains the electronic properties of solids, particularly how electrons behave in materials. It describes energy levels in solids as forming bands, with the highest energy band being the valence band and the next higher band being the conduction band. The gap between these bands is called the band gap, which determines whether a material is a conductor, insulator, or semiconductor. In conductors, the valence band and conduction band overlap, allowing electrons to move freely. In insulators, a large band gap prevents electron movement, while semiconductors have a smaller gap that can be overcome with energy, enabling controlled conductivity.