Osmosis takes place by the diffusion of water molecules through a plasma membrane. The membrane controls the flow of objects moving through it, and most polar molecules such as sugars and proteins cannot move through the plasma membrane, even though water molecules are polar, they are small enough to move through the membrane freely. If a molecule is on one side of the membrane and it is large and polar it will not be able to pass through the membrane, but it will interact with the other polar molecules, such as the water.
If the large, polar molecules are only on one side of the plasma membrane, then more water molecules will move to that side, making the amount of water molecules uneven, when it has unequal osmotic concentrations, the side with a higher level of solutes id hypertonic, and the level with the lower level of solutes is hypotonic. When the concentrations of solutes are equal on both sides, the solutions are reffered to as isotonic. The water molecule's object is to keep both sides of the membrane as isotonic as possible.
One of the main examples of the use of osmosis in nature is in plants. Osmosis is usxed in plants to get water out of the ground, but there are two important factors in the plants using osmosis effectively, they are osmotic pressure and osmotic gradient. Osmotic pressure is the force that raises the turgor pressure on the cell wall and helps create an even flow of molecules. Osmotic gradient is the difference between the two solutions concentration, it helps with the osmotic pressure.
There are two different variations of osmosis. Reverse osmosis is a process that uses the application of pressure on one side of a membrane to put a solution through it and keeps the solute on one side and moves the solvent to the other; this process is typically used for water. Then there is forward osmosis, which is alot like reverse osmosis only it uses osmotic pressure gradient, instead of hydraulic pressure, and a draw solution of higher concentration is used to bring the solution through the membrane, separating the water from it's solutes.
If the concentration of a solution on one side of the plasma membrane is to rapidly change, it will induce osmotic shock, or osmotic stress, resulting in rapid movement of water through the cell membrane to counter-act the difference. Osmosis is a very fine-tuned process that helps millions of organisms keep hydrated.