Dont get confused between 'types' of water.
The most common and easy to get is 'softened' water.
It used a resin bed to remove the calcium ions in the 'hard' water and replace them with sodium ones.
It means water will dry with very few mineral deposits and is cheap and easy to do.
The next step is de-mineralising, which is done by two methods.
Distilling (hence distilled water), where the water is boiled and condensed, or by Reverse Osmosis where a semi-permeable membrane filters out the minerals.
The final and most expensive step is de-ionising, where EVERYTHING is removed, producing pure water.
This uses Anion and Cation beds to extract both the + and - ions present in the water, leaving it as close to pure as its going to get.
The purity of water is usually measured by its conductivity, expressed in Siemens, milli-siemens or micro-siemens.
The closer to 0 it is, the purer it is.
I've used softened water to wash my car, and it works great, tho never had a chance to try de-ionised.
Most power stations have a de-min plant to provide boiler feedwater free of anything that will leave deposits in the tubes when its flashes off into steam.
