A traditional Water  Softener removes hard elements from your water like limestone  and manganese.  It also removes heavy metals like iron and lead.  These elements are searching for a positive electron, thus attaching to the cation positively-charged resin in the  tank of the water softener. In the presence of a strong brine solution, the resin will release the hard elements in exchange for sodium. This occurs in the regeneration cycle when hard elements flush out the drain. 

NO SALTWATER  SOFTENERS – Do not remove anything like iron or lead. They change the structure of calcium so it supposedly won’t scale. This is effective for a short time and the media in the tank needs to be replaced every 3-5 years at a cost of ~$800. Because the media is expensive, these units are sold with an in-out valve instead of a back-washing rinsing valve that removes dirt that is caught  in the filter.   
There are also electronic devices that can cause a change that prevents scaling.  This involves changing  bicarbonate to  calcium carbonate. The calcium will still leave spots but they can be easily wiped away rather than creating aragonite, which is hard-scaling, crystallized forms of calcium carbonate, CaCO₃.