Diffusion is the random movement of particles from an area of high concentration to an area of low concentration.
Osmosis is the movement of water molecules from an area of high water potential (or high solute concentration) to an area of low water potential (or low solute concentration).
The difference is probably that diffusion moves in the direction of decreasing solute concentrations, whereas osmosis moves in the direction of increasing solute concentrations. 🙂
