Most metered water softeners have a single resin tank, and separate brine (salt) tank.  Water softeners have a fixed capacity (based on size) to remove hardness from your water.  When that capacity is exhausted, the softener needs to regenerate in order to again provide softened water to your home or business.  Metered units (also called "On-Demand") measure the amount of water consumption, and only regenerate when necessary.  This saves both salt and water that is used during the softener regeneration process.  While costing slightly more than timer (clock) based softeners, metered units will lead to savings over time.  There are basically two type of metered water softeners:

Microprocessor Based Metered Water Softeners
These softeners measure water consumption using a turbine that turns as your water is used. An electronic sensor detects the rate of turbine spin and sends a signal to the on-board computer control. Using parameters that are pre-set by the user (capacity, water hardness, etc.), the computer determines when regeneration is required and initiates the regeneration process automatically.

Electro-Mechanical Metered Water Softeners
Similar to the microprocessor based softeners described above, electro-mechanical metered water softeners measure water consumption with a turbine that turns as your water is used. The turbine is connected directly to a mechanical water meter by cable. When the user-set gallon capacity is reached, an electrical motor is activated by a micro-switch, and moves the water softener through the regeneration process.

Time Clock Based Water Softeners

Twin Tank Water Softeners