We call it hardwater if it contains a lot of calcium, magnesium or other minerals. Groundwater acquires these metals by dissolving them from surrounding soil and rock. Industry measures water hardness in terms of grains per gallon (GPG) or milligrams per liter (mg/L). A grain is defined as 64.8 milligrams of calcium carbonate. If your water tests at 1 GPG (17.1 mg/L) or less, then you have soft water. Water around 1-3.5 GPG (17.1-60 mg/L) occupies a gray zone between soft and slightly hard water and 3.5-7 GPG (60-120 mg/L) is moderately hard. Hard water is around 7-10.5 GPG (120 - 180 mg/L), and very hard water is above that.
How do all those number affect you? Hard water causes two problems:
You can combat hard water in various ways, including filtering it by distillation or reverse osmosis, adding a packaged chemical softener such as powdered borax or washing soda (sodium carbonate), or running it through a water softener.
Filtration in sink taps and refrigerator water dispensers improves water's taste, but its steep price tag makes it impractical as a household solution. Packaged chemicals soften water in small batches, such as washing machine loads, but render the water undrinkable, take a toll on clothes, and, in some cases, contain phosphates that harm the environment.
Descaling offers an alternative to water softening. Whereas a water softener removes the problem (minerals in the water), a descaler addresses the damage caused by the problem (scale buildup). You will sometimes see ads for "salt-free water softeners," which are actually descalers, or for magnetic water softeners, which remain unproven and don't change the chemical composition of water, so buyer beware.
With all this in mind, it's clear why water softeners are so popular: They remain the least costly and most effective way to rid your water of troublesome minerals.