By far the most common type of residential water softener is the ion-exchange
system. Understanding the function of each part will help you maintain it correctly. 
Ion-exchange water softeners have three main components: a mineral tank, brine tank, and a control valve. Smaller capacity models combine the mineral tank and brine tanks into one cabinet, but the two tanks are still separated within the cabinet.
Larger flow capacity systems have a separate stand-alone mineral and brine
tanks:
 Mineral tank: The mineral tank is where the action is. It is where the water
filtration takes place and the hard water is softened by removing calcium and
magnesium.

 Brine tank: The brine tank is where a highly concentrated solution of salt or
potassium is stored. This brine solution comes into play to flush the mineral tank
and recharge it. The brine tank must be periodically replenished with salt or
potassium pellets. 

 Control valve: The control valve is the device that controls the flow of water into
and out of the mineral and brine tanks during regeneration.

In most instances, a water softener is located near the point where the water
supply enters the house, and is installed so it treats the water used for drinking,
cooking, and washing, but not the water used for outdoor irrigation.