Conduction Band

AT O^O C the outermost electrons of any atom of a semiconductor material (like silicon and germanium) are exist in valance-orbits. Connected with these orbits is a band of energies which is now termed like valence band of energy.

Band Gap

The difference of energy among conduction band and valance band is called upon band gap energy. 

In the given figure upper line is mention as conduction band whereas lower line is valance band. In these energy bands there are not any electrons available.