Water as a chemical is both familiar and peculiar. Because water is by far the most common liquid we interact with, we do not appreciate its unusual properties. The topic of water is extensive and can only be sampled here.
Water is a molecule with two hydrogen atoms attached to one oxygen atom, written H-O-H or H2O. That it freezes at 0°C and boils at 100°C was the basis for the Celsius or centigrade temperature scale. Water is somewhat dense for a liquid at 1 gram per milliliter (again, the basis for the definition).
Water being dense gives it force when it moves in rivers and oceans. Water power from dams and tidal generators provides significant energy.
Another unusual property of water is its heat capacity. Raising the temperature of water requires a large amount of heat compared to most substances. Phrased differently, water cools off slowly. This has many consequences. For example, cities on a coastline experience mild summers and winters because the ocean acts as a buffer against large temperature changes.
The water molecule is shaped something like the letter "V" with both hydrogen atoms (white in the diagram) on the same side of the oxygen (red). Because of the way atoms in a molecule share electrons, the oxygen side has a negative electrical charge and the hydrogen side is positive.
Water will dissolve an extremely wide variety of substances. It has been called "the universal solvent". Because of this, the chemistry of water affects every aspect of our lives and our environment. It is great at cleaning, and the addition of soap makes it better. Water in the ocean has collected salt, phosphates, and traces of almost every element on Earth. Water in our bodies carries nutrients, chemical signals, and waste products.
Water expands when it freezes, as we all know. Most of us don't realize how unusual this is, though. Solids typically contract as they freeze and sink to the bottom of their liquid form. As ice forms instead at the tops of lakes and oceans, it is exposed and available for melting in warmer weather.
Ice is common elsewhere in the solar system. All of the giant planets (Jupiter, Saturn, Uranus, Neptune) have ice in their atmospheres. Most of the moons of the outer system consist of ice, as do the rings of Saturn. Comets are primarily ice.
A considerable amount of energy is needed to boil water at 100°C into steam, and the same amount of energy must be extracted to condense steam back into water. A steam engine exploits this principle. Water below the boiling point also turns slowly into vapor, a process called evaporation. Evaporation goes faster in warm, dry air than in cold, moist air.
Ice also turns slowly into vapor, skipping the liquid phase. Because of the low temperature, this is a much slower process, called sublimation, than is evaporation. An easy example of sublimation is ice cubes in a freezer that gradually shrink.
Steam or water vapor is a major component of the atmosphere, although the percentage (called "humidity") varies greatly by location. Water vapor is a gas, less dense than oxygen or nitrogen, and so it tends to rise. It is a greenhouse gas.
Clouds are not water vapor, which is invisible. Air can hold only a certain amount of water vapor, and this capacity decreases as the temperature drops. Clouds form when water vapor condenses into visible but tiny droplets, collecting on particles of dust or salt in the air. When these droplets grow too large to remain suspended, rain falls. Temperatures below freezing produce ice crystals and snow instead.
Seventy percent or so of our planet's surface is water. Water moving as rivers and rain carves and shapes, both because it is dense and because it dissolves. That which dissolves, such as calcium minerals, is carried to the seas. That which does not dissolve, such as basaltic rock, can still be smoothed and rolled.
Ice as well as water shapes the Earth's surface. Glaciers move slowly but with great force, scouring the land. Mountain peaks such as the Matterhorn in the Alps have been scalloped by high-altitude glaciers. The lakes of Canada and northern Europe were carved by glaciers. On a much smaller scale, water that seeps into rock crevices will expand when it freezes. This widens the crack and potentially splits the rock.
The two foundations of life as we know it are carbon and water. The carbon atom is the basic building block for complex molecules such as proteins. Water is the medium that dissolves, that is to say transports, the nutrients and active chemicals of life.
As we understand the evidence, life on Earth began in the oceans. The oceans are and were primarily saltwater, so when organisms with cells evolved, the cell enclosed fluid that was essentially seawater. Our salty blood is a descendant of this.
Humans require around 2-3 liters of water per day, depending on weight and activity. Plants and animals that have adapted to dry climates also need water, but they have evolved means of storing water or reducing losses. Scientists looking for life elsewhere in the universe look for water, assuming for lack of an alternative that all life needs water.