"Hard" water has a relatively high amount of dissolved mineral salts in it, primarily calcium or magnesium salts from limestone or gypsum aquifers. Properly, dissolved metallic salts such as iron or copper and others do not add to 'hardness' but cause the water to stain plumbing fixtures. In some cases there are also salts of aluminum or manganese. If you are in parts of Canada and some parts of New York state, these may be a concern.
Hard water is treated most commonly by replacing hard calcium salts with 'soft' sodium (less often potassium) salts. These salt-based ion-exchange systems are typically whole-house and treat all the water going into the house both for washing and cooking. There is an increasing amount of very good data suggesting that ion-exchange softened water is not particularly healthy *and* that hard water has some protective qualities against heart disease. But, on the other hand, ion-exchange systems are effective with very hard water when alternatives are not.
Other methods include what is known as "nano-technology" systems - useful only when hardness is very low, so low as to be almost negligible for practical purposes, "reverse osmosis" which is very effective but will not handle high volume applications, and finally "magnetic technology", otherwise known as smoke-and-mirrors or snake oil. Magnetic systems are very nearly useless even by the maker's own admission.
If I were to advise anyone on the use of water softeners, I would suggest that they were useful for flushing, washing and cleaning in general, for the filling of heating systems, automotive radiators or similar applications. Extremely hard water would also suggest that one would _not_ use softened water for watering plants as the salt content would be quite high. And if one were on a septic system with a leach field large amounts of salt could affect ground water and so forth. But for cooking, drinking or any other food use - even the watering of plants - un-softened water would be best and most healthy. I have done a couple of separations for friends - running home-runs from the service to the kitchen sink cold water side and hose bibs - understanding also that one should _never_ use hot water for cooking anyway. This would be good for a dishwasher as well, by the way as it is fed exclusively with hot water.
Hope this helps.