As water passes through the atmosphere in the form of rain, snow, sleet, hail, dew, or fog, it picks up impurities and gases. And, because water is the universal solvent – that is, dissolves a part of whatever it comes into contact with – it picks up even more impurities as it travels through the earth.
In particular, the minerals calcium and magnesium are water "hardness" culprits. If either or both are present in your drinking water in high concentrations, the water is considered "hard". Those deposits are what appear on your dishes, clothes, sinks, and fixtures.