Typical water treatment used by Illinois citizens:
Physical filter -- Usually composed of paper or cotton-like material. Used to remove visible particulate matter from water.
Iron filter -- The most efficient units use an oxidant such as chlorine or potassium permanganate to precipitate dissolved iron from the water. May be used in addition to a softener or other treatments.
Carbon filter -- Contains charcoal or a similar material to adsorb pesticides and odors. Should not be used as a physical filter.
Softener -- Removes calcium and magnesium, the major hardness components, and replaces them with sodium. Regenerates with sodium chloride on a routine basis. Eliminates scaling inside pipes and plumbing fixtures. Click here for more information.
Reverse Osmosis/Distillation -- Demineralizes (removes all materials from) the water. Appropriate for treating excessive nitrate, sulfate, chloride, fluoride, etc., in water (when proper construction methods cannot keep the substances out of the water).
Chlorinator -- Establishes a chlorine residual in the water that can be used to oxidize iron or hydrogen sulfide (rotten-egg odor) gas, or kill bacteria. Should only be used when proper construction methods cannot keep these materials from entering the water initially.
US EPA Treatment Database
The US EPA has a "Drinking Water Treatability Database" where you can read more about many treatment techniques. You can search by either contaminant name or a particular treatment process. Follow this link for more information: USEPA Treatment Database