Most forms of matter can exist in a solid, liquid or gaseous state. What chemical state matter exists in at any point in time depends on the energy levels of the individual molecules that compose the matter itself. Solid matter is characterized by low energy levels, liquids by medium energy levels, and gas by very high energy levels. These energy levels will determine the strength of the bonding between the different molecules and, consequently, the state of matter.
When water is in a liquid form, its individual molecules are too highly energized to conform to a solid structure, but not energized enough to break apart from one another entirely. When water molecules gain enough energy to break their bonds with other water molecules, they evaporate and become a gaseous vapor. While evaporation is a constant process, unless it is hot enough to energize many molecules at once, it usually is a very slow process.
One water has evaporated into a gaseous state, it becomes part of the air people breathe in every day. It can either rise up into the sky and help form clouds or it can re-enter a liquid water state by coming in contact with a surface cold enough to deprive it of energy. This process is called condensation, and people see it in action whenever they notice a cool glass that was dry on the outside is suddenly covered with droplets of water.
The primary mode whereby evaporated water molecules return to the earth in a liquid state is through rain. Water vapors will form into clouds in the sky, and weather conditions will eventually cause the individual vapors that compose those clouds to condense into liquid form. When enough of the vapors are condensed at once, the result is water falling from the sky in the form of a rain shower.