The simplest type of passive transport is diffusion or simple diffusion. Osmosis, which is also a form of passive transport, is similar to diffusion in that both move a substance from an area of high concentration to an area of low concentration.
Diffusion and osmosis do not require a cell's energy to move a substance. The difference is that in diffusion, the substance been moved is a solute, whereas in osmosis, it is water.
In diffusion, the movement of a solute continues until its concentration reaches equilibrium or eliminates the concentration gradient – that is, the concentration difference between the two areas. In osmosis, water molecules move across a semipermeable membrane until the concentration of both the water and solute are equal; in other words, the dilution of the solute particles has to occur.