Drinking water Drinking water is just that: water that is intended for drinking. It is safe for human consumption and comes from a municipal source. There are no added ingredients besides what is considered usual and safe for any tap water, such as fluoride. (Incidentally, my tap water in New Jersey didnt even contain fluoride a necessary mineral for a childs growing teeth and gums. We had to give our kids fluoride supplements.)
Distilled water Distilled water is a type of purified water. Its water that has gone through a rigorous filtration process to strip it not only of contaminants, but any natural minerals as well. This water is best for use in small appliances like hot water urns, or steam irons, because if you use it, you wont have that mineral buildup that you often get when you use tap water. Though it may seem counterintuitive, this water is not necessarily the best for human consumption, since all of the waters natural, and often beneficial, minerals are absent.
Purified water Purified water is water that comes from any source, but has been purified to remove any chemicals or contaminants. Types of purification include distillation, deionization, reverse osmosis, and carbon filtration. Like distilled water, it has its advantages and disadvantages, the advantages being that potentially harmful chemicals may be taken out and the disadvantage being that beneficial minerals may be taken out as well.
Spring water This is what you often find in bottled water. Its from an underground source and may or may not have been treated and purified. Though spring water sounds more appealing (like many others, I imagine my spring water coming from a rushing fresh spring at the base of a tall snow-capped mountain), its not necessarily the best water for drinking if you have other options. Studies done by the NRDC (Natural Resources Defense Council) have found contaminants in bottled water such as coliform, arsenic and phthalates. Much of bottled water is labeled as spring water, when in fact it is coming from a municipal source and is nothing more than glorified tap water. This topic has been a popular one in recent years, sparking much controversy.