Conductor: Has a very small or negligible band gap, allowing electrons to move freely.
Insulator: Has a large energy gap (typically greater than 3 eV), which prevents electrons from jumping from the valence band to the conduction band, resulting in very low electrical conductivity.
Semiconductor: A moderate energy gap (about 1 eV) allows some conductivity under certain conditions (like temperature or doping).
Intrinsic Semiconductor: A type of semiconductor with a pure form, that also has a moderate energy gap similar to other semiconductors.