Water softener replaces the dissolved calcium and magnesium (which are responsible for hardness) in water with either sodium from sodium chloride (table salt) or potassium from potassium chloride salt. This is a system typically used for equalizing drinking water back to a point it is neither hard or soft, and thus, a higher quality water for drinking or washing clothes.
This is different from a water filter system, in that filters will generally remove bacteria (in some cases), suspended particles (sand, sediment, etc.) and specific chemicals, depending on the filter media. Most filter systems do not remove dissolved solids, such as those that cause hardness. Water filter systems are primarily concerned with trapping bacteria or solids which could produce bacterial conditions in those who drink from it.
Newer pressure filtration systems capable of nanofiltration and reverse osmosis filtration will remove salts, and reduce hardness. A descaler does not remove dissolved solids; it attempts to prevent the solids from being deposited inside pipes, on fixtures, etc. The word conditioner is very vague and is commonly used in reference to lime scale prevention. Water conditioners have not been scientifically proven to prevent hardness problems. As you can see the three systems are quite different and specific in their use.