Refraction is simply the bending of light when it moves from one material into another. If light is beamed at 90° to a surface, no bending happens. But if you shine the light at an angle it will bend one way or another.
Diffraction is a process in which a beam of light travels through a gap or around a barrier, and spreads out as a result.
Dispersion is the property that the speed of light in a transparent material is different for different wavelengths. Thus the index of refraction is likewise different.