Ion exchangers are used to remove salts (cations and anions) from the water.

Ion Exchange resins are insoluble granular substances which have in their molecular structure acidic or basic radicals that can be exchanged. The positive or negative ions fixed on these radicals are replaced by ions of the same sign in solution in the liquid in contact with them. removes dissolved metals such as Cadmium, Zinc, Lead and Nickel.

http://www.lenntech.com/library/exchange/ion-exchanger-principle.htm