Water is an essential part of the earth system. Water is special not only because it covers over 70% of the earth's surface, but also because it is the only known substance that can exist in gaseous, liquid, and solid phases within the relatively narrow range of temperatures and pressures found on earth.
Water's special qualities come from the unique shape of the water molecule. Each molecule contains two atoms of hydrogen and one atom of oxygen, arranged such that one side of the molecule (nearest the hydrogens) is positively charged while the other side (nearest the oxygen) is negatively charged. If two water molecules come together, the positive side of one is attracted to the negative side of the other, making the molecules cling together. This simple fact accounts for the high heat capacity, surface tension, cohesion, adhesion, and other characteristics that make water so important to the earth's biosphere.
In general, when considering the states of matter, solids are more dense than liquids and liquids are more dense than gases. Water is a bit of a contrarian in this regard.
When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape. Ice has a very regular pattern with the molecules rigidly apart from one another connected by the hydrogen bonds that form a crystalline lattice. These crystals have a number of open regions and pockets making ice less dense than liquid water. This is why ice floats on water. Ice forms when the temperature is below freezing (0°Celsius or 32°Fahrenheit).
When ice is warmed above freezing, it melts and becomes liquid water. As a liquid, the attractive forces between molecules weaken and individual molecules can begin to move around each other. Because the molecules can slip and slide around one another, water takes the shape of any container it is in. Despite the "hardness" of ice, the spacing of water molecules per unit volume is actually greater than it is for liquid water. Hence, ice is less dense than liquid water (which is why ice cubes float).
The third state of water is the gaseous state (water vapor). In this state, water molecules move very rapidly and are not bound together. Although we cannot see water in its gaseous state, we can feel it in the air on a hot, humid day. Commonly, water boils at a temperature of 100°C or 212°F, forming water vapor. Many people believe that the visible plume of steam from a boiling kettle is water vapor. However, the steam that you see consists of very small water droplets suspended in the air, while water vapor is the invisible gas that results when water evaporates. We can "see" water vapor through the electromagnetic eyes of infrared-sensing instruments.
Water cycles endlessly throughout the atmosphere, oceans, land, and life of planet earth, taking each physical state at one time or another.