Through a process of evaporation, evapotranspiration and sublimation water is continually pumped back into the atmosphere to fall again as rain, snow, hail etc.
This continual movement sees water change from a liquid to a gas to a liquid and sometimes into a solid (such as with snow and ice).
The whole process can be said to be powered by the sun (causing the evaporation) and as the water cools, the clouds form and it can rain or snow.
The water falls either directly back into lakes, marshes and oceans - or makes it's way to such places via streams, rivers or underground drainage systems. some water is eventually captured in what are effectively underground lakes or reservoirs.
For more information see Wikipedia or the USGS site