Osmosis is a naturally occurring process for a solvent,such as water pass through a semi-permeable barrier,the semi-permeable barrier or membrane allow some things to pass through it,but not others,in nature,the direction of flow through the membrane is from a less concentrated solution,such as freshwater to a more concentrated solution.Reverse osmosis is when the opposite occurs by the pressurizing the concentrated solution,the high pressure force the water molecules to pass from the salty seawater solution through the membrane to the fresh water.