Chemically, deionized water is exactly what its name implies, water with ions removed so that it is effectively neutral, according to myronl.com. Most water contains elements with a charge, such as calcium and sodium. The water is filtered to remove these substances through an ion exchange process.
A common method for testing the makeup of soil or industrial pollutants involves diluting the substance in water. Impurities in the water can produce false readings, reacting with an ionic compound you are trying to study. Deionized water is therefore preferred, although it is not totally purified. It may contain substances that lack an ionic charge.
Those who frequently detail cars and wash windows often use deionized water for a better quality shine than tap water provides, according to Portable Water Deionizers. Most tap water contains minerals which remain on surfaces after the tap water dries, leaving streaks. With deionized water, there is no need to hand-dry a surface, and it does not contain elements that can damage a waxy coating as tap water can.
According to Liquiguard Technologies, deionized water is vital to many industries that require water which does not readily react with other chemicals or leave residue. Optical lenses require very clean surfaces before the final coating, to prevent distortion. Computers often use deionized water in cooling systems to prevent residue buildup and bacterial growth. Cosmetics sometimes list deionized water as "aqua" among their ingredients, including it because it does not contain impurities that could cause undesirable reactions.
According to Jim Swenson of Argonne National Laboratory, deionized water may or may not be safe to drink. While deionization removes harmful minerals, it does not remove bacteria that might cause illness. Deionized drinking water should also be purified by other means. As all minerals have been removed, not just those that are harmful, it is not a source of the minerals the body requires. If it is used as drinking water, minerals need to be supplemented.
"DI water" should not cause erosion of metals, if it is used in connection with machinery for instance, because its average pH range is not low enough to cause much damage to metal.