is reverse osmosis, it forces wate runder pressure through a semipermeable plastic film or membrane with atomic size pores in it so only pure water molecules and things approx 0.0001 microns or smaller pass through it. RO
does 90 to 98% of the work so there will still be some dissolved solids present in the treated water.
or deionization uses plastic resin beads with ionic and cationic or positive and negative electrical charges on them to attract and cling to substances left in the water. It does the remaining 2 to 10% of the treatment.
Neither by itself is particularly good at some things so you really do need both.
Most people create their own problems by not following the basics and refusing to stock appropriate to their system. Gregcoyote 2013
SPS = Stability Promotes Success