THe Salt gives the resin(zeolite) beads a sodium Ion charge. This charge attracts the iron, calcium, and magnesium ions from the water. You are not really adding salt to the water. Most softeners with run on a per gallon basis anymore. Once they have run so many gallons through them the will recharge. This usually includes a backflush to fluff the resin bed a brine pull. Brine is the salt water from the brine tank. One it recharged the zeolite beads it will end witha rinse. To remove the brine solution from the main tank.
The RO Reverse osmosis unit will be made up of a prefilter to reduce particulants in the water. Then a carbon filter of sorts to remove order but the main reason is to remove chlorine, flourine and bromine from the water. Then it can go through a ro Membrane. This will remove whats left. This should leave you with about 95%+- water. You are basically removing the dissolved solids from the water.
Now some people have different iseas on DI and RO. From what I have learned over the past ten years in lab work and such. These are not the same. DI water is one step better. Somepeople have a DI unit on there RO system. DI or DeIonized Water is just that. Water with no Ions. Most of the DI water in the lab where I worked was about 99.9 % free of dissolved solids.
Like I said Some people think this is the same and I could be worng. But This is what I have gathered over the last ten years working with it.I am sure others have opinions as well. Thanks and I hope that this is helpful.