The carbon cycle is the movement of carbon from solid to liquid to gas, and from rocks to water to air to organisms.

Carbon dioxide gas, CO₂, makes up approximately 0.04% of the Earth's atmosphere. Plants take it up during photosynthesis to form sugars (chemical symbol C₆H₁₂O₆). During this process, plants give off oxygen as a byproduct. Animals consume these organic carbon molecules by eating plants and one another, and break them back down during digestion, returning some of the CO₂ to the air and using the rest to build tissues.

Organic matter that isn't consumed - feces, shells, wood, leaves, flesh, and all - accumulates layer upon layer on the seabed, at the bottom of swamps, along the forest floor. Eventually it hardens into rock or liquefies into petroleum. when the rocks weather or the fuel is burned, the carbon is liberated back into water, soil, and the atmosphere.

Atmospheric CO₂ can be absorbed by seawater as well. Cold water can hold the most, so the icy waters at the poles take up carbon dioxide and transfer it to the depths. When cold ocean currents rise up to the surface in tropical areas, the warm water releases CO₂ back to the sky.

Both human and natural processes emit carbon dioxide into the atmosphere. When humans burn fossil fuels (coal, oil, and natural gas), they release carbon into the atmosphere. Volcanic eruptions, cellular respiration from organisms, decomposition, and ocean water evaporation are all examples of natural processes that add carbon to the atmosphere.

Source: Hippocampus.org http://www.montereyinstitute.org/courses/Non-Majors Biology/course files/multimedia/lesson50/lessonp.html?showTopic=2