Softening by ion exchange resin is also known as Base Exchange softening. It is the most common and probably the easiest method of removing hardness like calcium & magnesium from water and renders the water suitable for utility purpose. As the name implies ion exchange is a process in which undesirable ions are exchanged for more desirable ions.
It consists of passing raw water containing hardness through a bed of cat ion resin in sodium form. The hardness ions Ca & Mg are taken up by resin and in exchange sodium ions are relinquished from the resin. Raw water will continue to get softened till the resin gets exhausted. Bringing back the resin to its original form is called regeneration. Softener resin is regenerated by sodium chloride.