The essential differences between surface, bulk and grain boundary diffusion?

I have not heard of the terms "surface diffusion" or "bulk diffusion", so I can only guess that they occur only on the surface of a material, or within the entire bulk of a material. Grain boundary diffusion occurs when crystals are subject to large stresses from regional metamorphism, causing atoms to dislodge from the crystals and flow away from the pressure, usually redepositing in areas of less pressure. The resulting features sometimes are confused with sigma clasts.