When two saline solutions are separated by a semipermeable membrane, water will pass through the membrane from the side with lower salt concentration to the side with higher salt concentration. This spontaneous passage of water is called osmosis. The driving force of the osmosis process is the gradient of the salt concentration across the membrane. The water passage across the membrane continues until the salt concentration of the water is equal on both sides, which is called equilibrium. If pressure is applied to a membrane in contact with a salt water solution and that pressure exceeds the osmotic pressure from that salt water solution, the water flow can be reversed leading to the extraction of pure water from the salt solution. This process is termed reverse osmosis (RO). 

RO membrane is a semipermeable membrane, which is permeable to water but not to other species, such as dissolved salts.  A typical composite RO membrane is a three layer structure with the selective top layer and the porous support layer usually made of different polymeric materials. The selective top layer is formed on the porous support, typically by an interfacial polymerization reaction. RO membranes can be further classified according to their applications as either seawater (SW) membranes or brackish water (BW) membranes.