Their are two types of water hardness: temporary and permanent hardness. This page gives you information about the temporary water hardness. For information and calculation of the permanent hardness click here.

Temporary hardness is due to the presence of calcium hydrogencarbonate Ca(HCO₃)_2(aq) and magnesium hydrogencarbonate Mg(HCO₃)_2(aq).
Both calcium hydrogencarbonate and magnesium hydrogencarbonate decompose when heated. The original insoluble carbonate is reformed. This happens when water is boiled. The precipitation reactions are as follows:

As you can se boiling the water causes the precipitation of solid calcium carbonate or solid magnesium carbonate. This removes the calcium ions or magnesium ions from the water, and so removes the hardness. Therefore, hardness due to hydrogencarbonates is said to be temporary.

Generally an increase in water temperature causes an increase in the solubility of most salts. But as you may have understood from above there are exceptions like CaCO₃, CaSO₄, MgCO₃, Mg(OH)₂ all of which become less soluble as the temperature increases.

Go to the permanent water hardness page.