Reverse osmosis (RO) is when we use the osmosis phenomenon "backwards" by increasing the pressure of the unfiltered water, it will be forced through a semipermeable membrane which will let through only the water molecules. A major advantage with reverse osmosis water filtration is that no chemicals are used in the drinking water.

Osmosis was known as a natural phenomenon as early as in the 18th century. In the nature, water is purified and transported through osmosis - in the body of mammals as well as in plants. In a living organism, the cell wall is the membrane through which nutrients and foods are transported via osmosis. Reverse osmosis for water purification and other uses was invented much later.

If you drink salty water it will eventually kill you. Osmosis is to blame for this - since the salty water on one side (in your stomach) is separated from pure water in your body by the stomach wall, the osmosis process will lead to your body being dehydrated in an attempt to "dilute" the salty water in your stomach.
When using reverse osmosis, the general idea is to use the membrane like a very fine filter for the creation of drinking water from contaminated (salty - or other contaminants) water. By applying pressure on the contaminated side, it is possible to reverse the osmosis process and achieve filtration. It is a slow process but it works well.

Membranes used in the reverse osmosis purification process are not using pores but are instead using very dense layers of polymers of microscopic thickness. The membrane used will allow only water to pass through. For the whole thing to work, it is necessary to use a fairly high pressure on the contaminated side of the membrane.

Reverse osmosis is otherwise best known as a desalination technique. Typical applications would include but are not limited to drinking water, humidification systems, produce watering and spa systems.