Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. Imagine a semi permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to fresh waterside.
A very important part in the packaged drinking water treatment system, reverse osmosis is also often used in commercial and residential water filtration. It is also one of the methods to desalinate sea water.