Deionized water can be created by running distilled water over a bed of beads which have an ion-exchange resin. The ions in the water are then bonded to the resin. This process filters the mineral salts out of the water, leaving a very pure form of the liquid. There are variations within this process, with some approaches repeating this method a certain number of times. There also are considerations in this realm regarding the repeated use of the same equipment, which tends to lessen in effectiveness over time.
Although the majority of the dissolved salts may have been removed from the water, it can still contain other dissolved substances. Deionized water, while devoid of all or nearly all of its ionic load, may still contain some dissolved solids or organics. It may also still include viruses or bacteria.
Deionized water is used in many scientific fields or studies largely because ions in water can adversely affect some intended results. However, in many cases in the scientific realm, it may first need to be treated to increase its purity even further. Another reason that deionized water may be utilized is due to it not conducting electricity as well as other types of water, which may have more ions.
Deionized water is used with some car batteries to ensure or extend battery life. It is also used as part of a rinse at some car washes. Some other products in which it may be used include cosmetics and cleaning supplies.
A somewhat controversial subject is deionized water's suitability for drinking. Drinking any form of deionized water which isn't labeled for human consumption, especially those made solely for industrial use, could lead to harmful consequences. Deionized water is stripped of many minerals, but, among those who drink it, the lack of these can typically be made up for, given a healthy diet. As always, if one has imminent concerns regarding such a subject, they should ask a medical professional.