Even though there is water all around us in abundance, much of it is not drinkable or usable in its current state. That is because not only is a lot of our water contaminated, some water has excess salt or minerals as well as harmful bacteria. Two processes, deionization and demineralization, help to make water usable. These terms are often confused with each other but usually done together.
Deionizing water is a process that uses ion exchange resins to remove ionized salt from the water. This process can theoretically remove all traces of salt from water. In practice, this process also removes harmful things such as viruses, bacteria and organic materials. Commercial grade deionization equipment also not only removes the ionized salt molecules, sodium, but they replace them with hydrogen to soften the water.
Demineralization is often a term used interchangeably with deionization. Demineralization is essentially removing all the minerals that can be found in natural water. This process is usually done when the water will be used for chemical processes and the minerals present may interfere with the other chemicals. All chemistic and beauty products have to be made with demineralized water for this reason.
Deionization and demineralization, while being similar, have key differences. Deionization removes ionized salt from the water. Demineralization removes minerals from the water such as calcium, magnesium and many others. Much of the drinking water that we consume goes through both processes.
References and ResourcesLenntech: Deionised/Demineralised Water
Free Drinking Water: Water Deionization Process Explained