Osmosis: It refers to the flow process for the water molecules in dilute solution to spontaneously penetrate to the semipermeable membrane and enter into the concentrated solution side. 

Reverse osmosis. It refers to the flow process that  the operating pressure is applied on the concentrated solution side to overcome the natural osmotic pressure, when the operating pressure is higher than the natural osmotic pressure, the water molecules will reverse flow, and  the water molecules in concentrated solution will penetrate the membrane and enter the dilute solution.