If an atom in an intrinsic semiconductor (SC) is excited thermally so an electron jumps from the valence band to the conduction band, then the electron is free and the atom becomes a positive ion. This is called a hole. If an electric field is applied across the material, free electrons will drift to carry a current. If a free electron created elsewhere in the lattice collides with the hole they recombine into a neutral atom--but there is a hole left where that electron came from. It looks like holes drift too, opposite to the electrons. Because they are charged positively, they also contribute to carrying current. Notice that the ion itself doesn't move, because it is bound in the crystal lattice. It's the hole that moves. That's the difference.
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