Over fifth of the water sources in the world are underground water sources, or Aquifers. In an Aquifer water flows through a layer of rock, absorbing some of it on their way.
Once water has reached your tap, it contains residues of elements the rock layer was made of, usually Calcium and Magnesium (that is of course if your region relies on such water sources).
Such water are commonly referred to as Hard water.
Water which were treated, either industrially or domestically, and had the rock residues removed from, are commonly referred to as Soft water.
Hard water wreaks an invisible havoc on your little piece of heaven.
The stone-like residues have a tendency to accumulates on indoor plumbing systems and home appliances, such as laundry machines, either clogging them, or greatly decreasing the water flow pressure through them. This is a process of gradual deterioration, and given the fact that it takes place over a period of a few years, you might not even notice it.
The more obvious effect of Hard water is its reaction to detergents. When coming in contact with soap, for example, Hard water would react to it and leave an oily residue on your skin, rather than washing it thoroughly away. The same thing happens with laundry detergents, which is why most people use laundry softeners, without even knowing the reason (my dear mom: It's supposed to make the clothes smell good, isn't it?). In fact, laundry softeners perform the same role as a water softener.
Health, of course, is another major concern about Hard water. Many researches, published since the 1960's, suggest that there is a connection between Hard water, and the high levels of Magnezium it contains, and certain heart diseases. Though many claim any such connection is purely statistical.
Say you found out your water are Hard, what do you do?
Most people would be indifferent about it, and will regard it as the responsibility of their water company.
Others will choose not to rely on a major corporation to solve their problem, and will install a Water Softener in their homes.
Medical source: Magnesium, Water Hardness, and Heart Disease - T.W. Anderson (http://www.execpc.com/~magnesum/anderson.html)