Tap water has fluoride. That's good for your teeth.
It also has chlorine, chloramines, phosphates (especially in the spring and summer), Nitrates, various organic compounds, most likely some heavy metals, and according to some recent reports, traces of pharmaceuticals as well. Many of the nasties are seasonal, and regional as well. Farming is a major source of some of the chemicals in the water, even if there aren't farms specifically close to you.Tap water is cheap. The taste varies with region as well, but I've never really liked the taste of most tap water.
Bottled water comes in many forms:
Filtered drinking water-usually just tap water run through a carbon filter. You can do this at home if you wish.
Distilled-Tap water that has been steam distilled. It is very pure, and very safe to drink. Due to lacking phosphates and nitrates, it is good for fish tanks.
RO/DI water-Reverse osmosis and deionization. The purest water you can buy. Sometimes ozonated which sterilizes it as well. The ozone turns to Oxygen after use.
Spring water-must come from a spring and have a specific percentage of minerals in it. This does change the taste.
mineral water- has a higher percentage of minerals in it.