Reverse Osmosis is where water is forced through an incredibly small membrane (0.0006 micron) which allows water to pass through, but does not allow dissolved contaminants through. This process occurs naturally in nature in Seaguls, whereby a semi permeable Membrane in the back of a Seaguls neck allows it to drink sea water by removing all salt and contaminents.
A similar process happens in a Reverse Osmosis Water Filter. For this reason the resulting water is of a very pure quality, removing very high levels of vitually all particles in the water.
In normal Osmosis, for example, water would pass both ways through the membrane, but with reverse osmosis this is not the case: