Hydrological Cycle

Water falls to earth as rain and becomes included into the groundwater (see Aquifer), or runs into standing bodies of water, where it remains for some time thereafter. Eventually most of the surface water returns to the oceans via rivers, where it evaporates and condenses forming clouds. Cycles of cloud formation, rain and terrestrial runoff characterize the hydrological cycle. Groundwater, on the other hand, can take centuries to exit from the aquifer and return to the oceans. The same is true for water that is trapped as snow in the polar regions. Only about 2% of all the drinkable water on earth is in liquid form, and 20% of that is contained in just one place, Lake Baikal, Russia.

Links
  http://www.unesco.org/science/waterday2000/Cycle.htm