Tap water containing elevated levels of ever-present calcium and magnesium compounds is considered "hard". The higher the concentration of these compounds the harder the water. The water is fine to drink but causes scale on glassware, fixtures and tub/shower enclosures.
The use of a water softener is the traditional way to eliminate these compounds. Remember, the process exchanges sodium chloride ions for the above mentioned inorganics therefore you shouldn't soften your entire household sytem only the hot water side. This helps protect your plumbing and just as important assures that anyone having a low sodium diet doesn't exceed their allowable daily intake. There are softening systems available that use potassium chloride for the exchange process which wouldn't affect dietary concerns significantly.
Other types of exchange treatments exist but may require more hardware/maintenance or may border on mystical magic. Have your water tested to determine the grains per gallon of hardness (lab reports in parts per million can be converted) and check locally or over the Web to locate a system sized for your needs. Hope this is helpful.