You've probably heard the terms "hard water" and "soft water" thrown around in discussions of home plumbing or bathrooms. The difference is in the mineral content of the water, which manifests itself in things like soap scum, mineral deposits on hardware and fixtures, and even the amount of bubbles you get when lathering up.
Soft water has a lack of calcium and magnesium ions. (To get technical, it's classified in the States as having less than 60 mg/l of calcium carbonate.) If you have soft water in your house, you'll find that it produces little to no soap scum in the bathtub or shower stall.
By contrast, hard water has a high mineral content of calcium and magnesium (61 to 80 mg/l of calcium carbonate). If you have hard water at home, you'll find that it may be difficult to suds up your shampoo or soap, and that limescale or calcium buildups form on surfaces water comes into contact with including tea kettles or faucets. To combat hard water, water softening is often used, in which certain mineral ions are removed with various methods.