The Hydrologic cycle is what most people know as the water cycle. It's the process of rain. The water evaporates from the worlds oceans and lakes and forms into the clouds that we useually see. The clouds travel across the sky picking up more water and becomeing heavyer and heavyer as they go along. At some point they become so heavy to the point where the water presipitates and starts back at the beginning.
Whatersheds are blocks in a water source. Humans, animals and plants can cause and inturupstion in a watershed. An example is the wolves in Yellowstone National Park. Wolves deacreased the population of deer in the park which cause animals to come back, plants to grow and the rivers behavior to change.