Band Theory explains the behavior of electrons in solids, particularly in metals, insulators, and semiconductors. It describes how energy levels of electrons form bands due to the close proximity of atoms in a solid. The two main bands are the valence band, which contains electrons involved in bonding, and the conduction band, where electrons can move freely, allowing electrical conductivity. In metals, the valence and conduction bands overlap, enabling easy electron flow. In insulators, there is a large gap between these bands, preventing electron movement. Semiconductors have a smaller gap, allowing for controlled conductivity, which is essential in electronic devices like transistors and diodes.