Cation and anion exchanges are polymer resins which contains functional group with a mobile ion which can be exchanged with other ions present in the solution to be treated

Cation exchanges are represented as $R-H_{z}$ which are treated with acid and hold $H^{+}$ ion and can be exchanged with cations and hardness products salts $Ca^{+2}$ and $Mg^{+2}$

Anion exchanges are represented as R(OH)2 which can be treated with alkali and hold

OH-ion and can be exchanged with anions of hardness producing salts SO4,-Cl