From Envirowiki

Water is the chemical compound H₂O (oxygen dihydride, 2 hydrogen atoms with one oxygen atom), and, along with carbon, forms the basis of all life on earth. It is also one of the major weathering forces, with precipitation contributing to erosion, and deposition of silts, sands, and stone.

Water is probably the second or third most immediate human need, after heat and air. Pure water is important for drinking, although a low level of some impurities is ok for drinking, and some minerals in water can be beneficial. Water also harbours many forms of life, including fish and amphibians, insects and other invertebrates, as well as algaes and microbes. Some of the microbes and invertebrates can cause health problems, or diseases, so some way to sanitise the water is often required, especially water with lots of organic wastes, or pollutants.

Boiling water for a few minutes can get rid of most dangerous microbes, but can actually concentrate the levels of non volatile and sedimentary chemicals. - to remove these, the water must be filtered.