Semiconductors distinguish themselves from metals and insulators by the fact that they contain an "almost-empty" conduction band and an "almost-full" valence band. An energy band which is completely filled carriers no current. But if some of electrons in a filled band are excited into a higher band, the remaining electrons in it can contribute to current.

Holes are missing electrons. They behave as particles with the same properties as the electrons would have occupying the same states except that they carry a positive charge.The properties of vacant state in an almost full band are exactly those of particle with a positive charge in an otherwise empty band. This quasi-particle is known as a hole. It moves in an opposite direction to that of an electron.

Fig - Energy band diagram in the presence of a uniform electric field. Shown are electrons (red circles) which move against the field and holes (blue circles) which move in the direction of the applied field.