Water is a transparent and almost colorless chemical substance, which is the main component of the earth's streams, lakes and oceans as well as the fluids of most organisms. Its chemical formula is H2O, indicating that its molecule contains one oxygen and two hydrogen atoms, which are connected by covalent bond. Water strictly refers to the liquid state of the substance which exists generally under the standard ambient temperature and pressure. But it also generally refers to solid (ice) or gas (steam or water vapor). It also occurs in nature as snow, glaciers, ice bags and icebergs, clouds, fog, dew, aquifers and atmospheric humidity.

Water covers 71% of the earth's surface. This is crucial for all known life forms. On earth, 96.5% of the earth's crust water is found in the ocean, 1.7% in groundwater, 1.7% in glaciers and ice sheets in Antarctica and Greenland, and very small in other large water bodies, 0.001% in the air as steam, clouds (formed by ice and liquid water suspended in the air) and precipitation. Only 2.5% of the water is fresh water, and 98.8% of the water is in ice (except for ice in clouds) and groundwater. Less than 0.3% of all freshwater is located in rivers, lakes and the atmosphere, and even less (0.003%) is contained in organisms and manufactured products. More water has been found inside the earth.

The earth's water circulates through evaporation and transpiration (evapotranspiration), condensation, precipitation and runoff, usually to the sea. Evaporation and transpiration promote precipitation on land. Large amounts of water are also chemically bound or adsorbed in hydrated minerals.

Safe drinking water is essential for humans and other forms of life, even if it does not provide calories or organic nutrients. In the past few decades, access to safe drinking water has improved almost everywhere in the world, but there are still about one billion people without access to safe water, while more than 2.5 billion people lack adequate sanitation. There is a clear correlation between access to safe water and per capita GDP. However, some observers estimate that by 2025, more than half of the world's population will face water-based vulnerability. A report released in November 2009 shows that by 2030, water demand will exceed 50% of water supply in some developing regions of the world.

Water plays an important role in the world economy. About 70% of the fresh water used by human beings is used for agriculture. Fishing in salt and fresh water is the main source of food in many parts of the world. Long distance trade in commodities (such as oil and gas) and manufactured goods is carried by ships in the sea, rivers, lakes and canals. In industry and home, large quantities of water, ice and steam are used for cooling and heating. Water is an excellent solvent for many chemicals; therefore, it is widely used in industrial processes as well as cooking and washing. Water is also important in many sports and other forms of entertainment, such as swimming, boating, rowing, surfing, sports fishing and diving.