To begin answering this question, some basics: Carbon is one of the most common elements on Earth, occurring in many different forms in isolation as pure carbon and very often in combination with other elements to create a huge variety of types of matter. All living things in Earth depend on carbon as a basic building block.
The carbon cycle refers to the way in which carbon changes form as it is recycled from one use to another, from one compound to another. Because the cycle has no specific starting point or stopping point, we will join it in progress.
1. Earth's atmosphere contains large amounts of carbon dioxide. Photosynthesis by plants uses carbon dioxide from the air, absorbed into their leaves and converted to glucose and other sugars, to store energy as food.
2. When animals eat those plants, the carbon (now in the form of glucose) enters the animal's system. If an animal eats another animal that ate plants, the glucose (carbon compounds) is passed on.
3. When animals breathe, what they exhale is carbon dioxide. The carbon dioxide is released when the animal's body breaks down the glucose into usable energy. This means that this carbon has completed the cycle - it is now back in the atmosphere.
4. When plants or animals die, their physical remains undergo decomposition. "Microorganisms that live mostly in the soil but also in water" eat the rotting flesh or plant material, releasing carbon dioxide through their respiration - more carbon returning to the atmosphere.
5. Some carbon is turned to rock, as happens when animals with shells die and the shells are compressed. As these rocks are later exposed to erosion or weathering, the carbon in the rock is released back to the atmosphere.
6. When people burn fossil fuels such as coal (compressed carbon rock) or oil (the end product of compressed decomposed organic material), carbon dioxide is released into the atmosphere. Because the amount of fossil fuel being burned has greatly increased since the beginning of the Industrial Revolution, the amount of carbon dioxide going back into the atmosphere has also greatly increased. The "greenhouse effect" refers to the way in which this increased carbon dioxide has impacted the carbon cycle by upsetting the balance of nature's recycling process.