Snell's law relates the angle of refraction of light to the proportion of the two indexes of refraction. This angle is relative to the angle of the surface. So if you consider a ray of white light passing through a standard glass slab, the light bends when it enters the glass and then bends again when it leaves. But since the two planes are parallel to each other, the light that leaves the glass is at the same angle as the light that entered the glass - so all light is bent at the same angle and the separated colors are corrected as they leave.
With a prism, you will notice that they are never a regular shape. This is because when incident light enters it bends. All colors bend at different angles due to its own wavelength. When the light leaves the prism, the angle of the surface is different from the original one so the light gets bent again. Prisms are desingned to showcase the differnces in these refractions.
Hope this helps.