Osmosis is a type of diffusion that occurs across a semi-permeable membrane and tends to draw fluids from areas with relatively low particulate concentrations to areas with relatively high particulate concentration. Diffusion is the result of random processes, but osmosis occurs because of the chemical properties of the involved substances.
When a substance diffuses in the environment, the random motion of the individual molecules causes the substance to move from areas of high concentration to areas with low concentration. For example, if an individual introduces a drop of food color into a tank of water, the random movement of the food color atoms will cause them to spread out gradually. The initial droplet of food color contains a high concentration of the color, while the water has a very low concentration of the color. Over time, the molecules' random motion will tend to cause them to spread out evenly. When the molecules of the food color are evenly spaced throughout the water, the solution is said to be at equilibrium.
Osmosis most commonly refers to solutions involving water and salt. A solution that has a high concentration of salt is said to be hypertonic, while one with a relatively low concentration of salt is called hypotonic. When osmosis reaches an equal distribution of salt concentration, the sample is said to be isotonic.