The carbon cycle is the movement of carbon between the air (atmosphere) and land (biosphere). Carbon is constantly being exchanged back and forth between these two systems through a variety of mechanisms. Atmospheric carbon is mainly in the form of carbon dioxide and methane gasses. These gasses are extracted from the atmosphere into the biosphere via photosynthesis and the natural air/gas exchange with the ocean. Photosynthesis is the process by which plants absorb carbon dioxide from the air to produce food. Gas exchange in the ocean is the process whereby carbon dioxide from the air dissolves into the surface water of the ocean. CO2 in water produces a small amount of carbonic acid which gives the ocean a mild acidity.
Carbon in the biosphere is contained as a mixture of organic carbon (living organisms) and inorganic carbon (minerals). The combustion of fossil fuels and wood is one way that carbon is transferred back to the atmosphere. Also natural respiration by animals produces CO2. Decomposition and respiration of soil microbes is another method. Finally, the previously mentioned air/gas exchange with the ocean allows excess CO2 to be reabsorbed by the atmosphere.