Water softeners are systems used to tone down the hardness of water and make it usable for daily purposes. Salt based water softeners completely remove Magnesium and Calcium from water, the two major salts leading to the hardness of water and lime scaling in your plumbing system.
Salt free water softeners use electro-magnetic pulses to neutralize Calcium and Magnesium making the water usable. These are often called water conditioners as they do not remove the salts from water rather just neutralize them.
If the hardness of water is not too much then water softeners usually do the job quite well. They are easy to install and use. You can find both over the countertop and under-sink models in the market. Salt based softeners require a bit more maintenance as you need to change the cartridges and salt regularly for the device to keep working at its maximum potential. Salt free softeners require less maintenance and are cheaper too.
More information at Soft Water Filtration