What is osmosis?
Osmosis is the result of diffusion across a semipermeable membrane from a lower concentration to a higher one.
If two solutions of different concentration are separated by a semipermeable membrane, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. Diffusion occurs when the spontaneous net movement of particles or molecules spreads them from an area of high concentration to an area of low concentration through a semipermeable membrane.
Osmosis is a process in which a fluid passes through a semipermeable membrane, moving from an area in which a solute such as salt is present in low concentrations to an area in which the solute is present in high concentrations.
The end result of osmosis, barring external factors, will be equal amounts of fluid on either side of the barrier, creating a state which is known as “isotonic.” - http://www.wisegeek.org/what-is-osmosis.htm
Excellent source with more details here:
Here is a video which describes how osmosis impacts onion cells placed into tap water and then into a salt water solution.
Hope this helps!
I think of osmosis as a process where water "rolls down hill" from where it is more concentrated to where it is less concentrated (the water concentration). I'll explain below.
If you have a salt solution separated from a pure water solution by a membrane (that allows water to pass, but not the salt), then the pure water will flow in the direction of the salty water.
This more concentrated water flowing in the direction of less concentrated water (water concentration is super high, so difference is small..but conceptually this is a good way to think about it).
Conversely, you can think of the pure water as trying to dilute the salty water...so the pure water flows into the salty water in a feeble attempt to make both compartments the same concentration (it can't do it because the salt can't move...but it will try).
This is osmosis...where water flows into a solution in an attempt to dilute the solution. The less tonic solution will flow into the more tonic solution (tonic basically means "things are dissolved" in the water).