Band structure refers to the range of energy levels that electrons can occupy in a solid material, particularly in semiconductors and insulators. It is crucial for understanding how materials conduct electricity. In a band structure diagram, energy levels are represented as bands, with the highest occupied band called the valence band and the next higher band, which is usually empty, called the conduction band. The gap between the valence band and the conduction band is known as the band gap. Materials with a small band gap can conduct electricity more easily, while those with a large band gap are typically insulators. Understanding band structure helps in the design of electronic devices like transistors and solar cells.