it separates it in to two streams :)
A more fancy pants answer is that desalinization is the process of extracting salt from sea water to make drinking water. An easy way to do it is to boil salt water and capture the water vapor as it leaves the boiling pot. When the vapor cools, it condenses and reforms into water drops that through coagulation reform into fresh water.
e.g. removing salts from seawater to produce drinking water
Many methods for water purification and seawater desalinization have been used for a number of years starting in ancient times with good old fashion distillation (boil the water and catch the condensate leaving the bad stuff behind). The leading method now is membrane based...reverse osmosis. Expensive to build, expensive to operate and maintain.
Reverse Osmosis uses pressure to force water molecules through a special membrane with very small pores that trap salts and other dissolved solids (retentate) and results in up to 99% pure water (solute). In low pressure home use, RO usually results in 80% or more waste...5 gallons of waste water are produced for every gallon of useable water. In high pressure systems (1000 psi or more) used in saltwater desalinization, recovery rates can exceed 90%. High pressure systems require alot of energy to run however.
New studies are underway to improve membrane efficiencies (particularly in energy use) include forward osmosis which actually uses an ionic salt process and then removing the special salts from the solute..or good water side..of the membrane process.
Carbon nano-tubes built into membranes and electrically charged to repel salt ions before reaching the membrane, and biomimetic membranes utlizing aquaporins in a similar charged fashion hold some promise to improving the efficiency of RO systems but are still in the theory and development stage.
One new process that is actually in commercial development is Capacitive De-ionization which uses a flow-through capacitor designed to eliminate dissolved solids from water using a small electrical charge.