Our water needs are completed from two water sources - groundwater (underground aquifers) and surface water (lakes, rivers, streams). Water cycle is the process which replenishes the water in these sources. Whenever there is rainfall, the water seeps down the surface and replenishes the ground water, then ones which fall to the water bodies (rivers, lakes, etc.) renew the surface water. The water which flows down and get collected also adds both to surface and groundwater. The heat evaporates the surface water and condenses in the clouds. This is the process by which water is renewed. This is called Hydrological cycle.