Diffusion is defined as the movement of the molecules from a region of its higher concentration to the region of its lower concentration. The diffusion takes place from cell to cell. No special type of membrane proteins is present and it is not a highly selective method.
Whereas the facilitated diffusion depends upon the nature of substance crossing the membrane, lipid soluble substances diffuse faster. A type membrane protein provides sites for the transfer of molecules cross the membrane without the expenditure of the ATP molecules. The facilitated diffusion is very specific as it allows selected substances, whereas the simple diffusion is not. In facilitated diffusion the rate of diffusion is maximum when the transporter molecules are saturated.