The most common methods of removing salt from water are distillation and reverse osmosis. Distillation refers to the process of purifying water through evaporation and condensation, while reverse osmosis uses a semi-permeable membrane to remove salt and other particles from water.
Distillation consists of boiling water to create steam. The steam passes into a condenser, where it is cooled back into purified water in a different chamber. Salt, as well as bacteria and other impurities, are removed when the water evaporates as steam. The process is energy-intensive and historically expensive when used on large scales, though it is used in some countries where energy costs are low.
Reverse osmosis relies on polymer membranes that allow water but not salt to pass through when the saltwater is subjected to increased pressure. The increase of pressure reverses natural osmotic pressure, which unchecked creates equilibrium between both sides of the membrane. Reverse osmosis is more commonly used than distillation on a commercial or municipal scale.
Water purification systems that use distillation and reverse osmosis are available for home use. Most distiller models are for counter top use and require several hours to purify a gallon or less of water. Reverse osmosis purifiers generally hook into your tap and are marketed to remove other minerals, such as fluoride, rather than salt.