They both involve substances traveling from high concentration to low concentration over a membrane, which is the definition of diffusion. Simple is passive transport where molecules are passed to a lower concentrated area naturally over an easily passable membrane. Facilitated is a type of active transport, which is used when molecules are too large to diffuse simply over the permeable or semi-permeable membrane. Usually this process is assisted by protein channels embedded in the phospho-lipid bi-layer that makes up the cell membrane.
