The Electrons in the atoms have an interacion with the potential force of the neucleus. In accordance with the quantum theory of particles, only few such states can exists where an electron can exist. These states have a fixed energy level and these energy levels are not continuous, they have a discontinuity in between them. As we move up the energy levels, the difference in the energy levels of the two successive levels keep on decreasing and later they seem to be in a band almost continuous, and now these electrons exists in the band. The valance electrons are present in the band called valance band and the band just above it is called conduction band. The energy difference between these two bands is called energy gap of the metal.