Hard water is what causes the white scale buildup on faucets, dishes, pots, pans, glass shower doors and windows. Hard Water can leave a detergent film on your freshly washed clothes.
Soft water doesn’t leave a detergent film on your freshly washed clothes or spots on your fixtures, glassware, and shower doors.
Whether a water supply is "soft" or “hard" is dependent on the presence of two highly soluble minerals: calcium and magnesium.
When calcium and magnesium are dissolved in the water, mineral build-up on contact surfaces, possibly plugging pipes and damaging water heaters, and decreases the effectiveness of soaps and detergents.