Hard water is water that has a high mineral content. Hard water forms as it passes through substances that easily dissolve to form salts. Two examples of such substances that would form hard water are chalk and limestone. As the water percolates through such substances, they dissolve into their components- calcium and magnesium carbonates. Hard water is often seen as the rings around bathtubs and the mineral deposits on dishes.
On the other hand, soft water is water that contains little to no ions, especially salts of magnesium or calcium (specifically, less than 85 parts per million of calcium carbonates). As rainfall falls, it is naturally soft. Soft water also naturally occurs where rainfall and the drainage basin are formed along hard, impervious and calcium-poor rocks. Hard water can be treated using water softeners to make soft water. Water softeners replace the calcium and magnesium ions with sodium ions.