The total amount of water on Earth stays relatively constant; however, it does move about, changing phases (solid, liquid, gas) and locations. The hydrologic (water) cycle is responsible for circulating water between the Earth's reservoirs. The hydrologic cycle not only transports and recycles water, it is also critical for maintaining habitable conditions on Earth.

The water cycle includes the processes of evaporation, condensation, precipitation, transpiration, infiltration, and runoff. It also includes the collection & storage of water in earth's reservoirs.
Evaporation: water changes to water vapor
Condensation: water changes from a gas to a liquid.
Precipitation: water falls to Earth as rain, snow, sleet, hail, etc.
Transpiration: liquid water from the leaves of plants changes to water vapor
Infiltration: water that seeps into the soil and can become groundwater
Runoff: water on the surface of earth that does not infiltrate the ground