Osmosis is an example of simple diffusion. Simple diffusion is the process by which a solution or gas moves from high particle concentration areas to low particle concentration areas. This is also known as passive diffusion. A drop of red dye turning water pink is an example of the simple diffusion process.
When red dye is dropped into water, the particles are concentrated. As time goes on, the particles move away from each other, gradually turning the water pink. Simple diffusion follows a similar pattern on a smaller scale.
Simple diffusion happens by random motion. This means that no facilitator is needed to diffuse the particles, and no energy needs to be expended to diffuse the particles. Simple diffusion requires a semi-permeable membrane to allow particle motion. This is because the semi-permeable membrane will prevent certain molecules from coming in while allowing others.
Osmosis happens when the concentration of a molecule is high inside a semi-permeable membrane, but the membrane only allows water in. Because the molecule can't leave the membrane to reduce the concentration, water flows into the membrane, causing it to expand and equalizing the concentration. However, the membrane usually allows the molecule of high concentration to travel.