Water Softeners - A water softener is a larger electrical /mechanical device which removes the chemicals in hard water areas which cause scaling: calcium and magnesium carbonate. These devices turn hard water to soft water.
Scale Prevention - These prevent these chemicals depositing themselves inside pipes, tanks and shower heads. These are small plumbed-in devices that use chemical, electrolytic or magnetic methods to accomplish this. Combimate which is a hard water scale inhibitor is in this group.
Water Filter - these remove bacteria, chlorine, pesticide residues etc. These mostly come in the form of handheld filter jugs with disposable cartridges for potable use.