Water is said to be soft if it has a low concentration of calcium and magnesium ions in it, while hard water has a high concentration of calcium and magnesium.
The minerals in hard water often react with most soaps to create a "soap scum," which is an insoluble substance that can temporarily adhere to your hands or the shower walls. When using hard water, you may have a harder time working your soap into a lather and you may need to use more water to rinse the scum from your hands. If you normally use hard water, you may find that washing your hands in soft water feels more slippery. This is because soap lathers more easily in soft water and less soap is required to clean your hands than when using hard water. The slippery feel of soap will go away more quickly with hard water when scum is formed, but that that doesn’t mean the soap (and scum) have been rinsed off of your hands.
Hard water is prevalent in some parts of the country. Sometimes a water softener is used to replace the calcium and magnesium ions in the hard water with sodium ions. The softened water allows soaps and detergents to be used without forming soap scum.
Learn more: Hardness of Water