The Hydrologic Cycle
The Hydrologic cycle, or, more commonly known as the water cycle is one of the three cycles of matter, essential for life. It cycles water in all of its forms (solid, liquid, gas) throughout the world. For example, water in the ocean can evaporate. The water vapor condensates to create clouds, which then rain onto the surface of a mountain. The snowmelt can infiltrate the ground, becoming groundwater, runoff into streams and freshwater reservoirs, or become surface runoff back to the ocean.
The Carbon Cycle
Carbon is the structure of living things. We known scientifically as carbon-based life forms. Carbon is present in both living forms (plants, animals, humans) and in nonliving forms (rocks, water, carbon dioxide (CO2), etc). The Carbon Cycle also contributes carbon as a gas to the atmosphere, adding to the Greenhouse effect (the heating of earth due to greenhouse gasses trapping heat within the atmosphere.
The Nitrogen Cycle
Nitrogen is also very important to life, as it is a major part of amino acids, proteins, and enzymes. Nitrogen in its atmospheric form, however is unusable to humans. Nitrogen fixing bacteria called Rhizobia that live in the root nodules of legumes (beans, lentils, peanuts) convert the nitrogen gas (N2, which is practically useless to humans) into ammonium.