Hard water is water that contains contains a lot of magnesium and calcium. This can cause unsightly soap scum, and makes it hard to lather when taking a shower. Even worse, it can clog up pipes, sometimes completely restricting water flow. This can be very costly to fix, and can be easily prevent with the purchase of a good water softener.
To put it simply, water softeners replace the calcium and magnesium with sodium ions. Most water softeners use salt, which contains sodium. This is why every so often you have to fill your water softener with salt.
To learn more check out this article at HowStuffWorks.com.
