Band structure refers to the range of energy levels that electrons can occupy in a solid material. In a solid, atoms are closely packed, and their electron energy levels combine to form bands. The two main types of bands are the valence band, which is filled with electrons, and the conduction band, which is typically empty. The energy gap between these bands determines whether a material is a conductor, insulator, or semiconductor. The concept of band structure is crucial in understanding the electrical and optical properties of materials. For example, in semiconductors, the energy gap can be manipulated to control their conductivity, making them essential for devices like transistors and solar cells.