Like air, water is a hard thing to quantify. As long as it isn't cloudy or strangely flavored, it seems interchangeable with any other. One way to separate hose water from the triple-distilled purified variety is by using a TDS meter, which measures the level of salts, minerals and anything alien in your water. On a TDS, American tap water can range anywhere from a dubious 700 parts per million to a relatively pristine 25. Some bottled water can even top 300ppm.
The ZeroWater filtration system, on the other hand, got its name for purifying tap water and consistently producing a zero on TDS meters. While comparable water filtration pitchers rely solely on chlorine, the ZeroWater system uses a five-stage ion-exchange filtration system that removes any discernible additives. And ZeroWater is so confident in their product that they include an independently-manufactured TDS meter with each purchase.