Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, while osmosis is the movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In other words, diffusion refers to the movement of any type of molecule, while osmosis specifically refers to the movement of water molecules. Both processes occur spontaneously and are driven by the concentration gradient of the molecules or solutes involved.