You know that the water on our planet is in constant movement. For example, water goes from seas to clouds, and from clouds to the surface of our planet. This movement is what we know as the water cycle.
The water cycle is the process in which the Earth's water moves between the hydrosphere, the atmosphere, the geosphere and the biosphere.
This is possible thanks to the energy our planet receives from the Sun and the gravitational pull. Heat causes water to change the state; gravity moves water from higher points to lower ones, in the seas. Here you have a picture with the cycle water.
|The water cycle (from USGS).|