Composting is simply the natural decomposition of organic matter. It is a process that is occurring constantly all around us. Compost is produced through the activity of tiny organisms known as decomposers. Given a favorable environment, they will break down your yard wastes and kitchen scraps into a humus-like material that can serve as an excellent soil amendment for your yard and garden.
Once you have established your compost pile, the decomposters go to work almost immediately. At one time or another, your pile will probably be populated by fungi, bacteria, protozoa, roundworms, flatworms, snails and slugs, various types of insect larvae, millipedes, bettles, mites, centipedes and more. Different organisms prefer different organic materials and temperatures; as the conditions in the pile change, the mix of organisms will change, too, with some organisms becoming dormant, dying, or moving to another, more hospitable, part of the pile.
Probably the most important thing to know about the organisms involved in composting is that the most desirable decomposers require oxygen. If your pile becomes oxygen deficient, these desirable organisms will die, and anaerobic decomposers (those not requiring oxygen) will take over. The anaerobic decomposers will generate odorous products as well as acids and alcohols that can harm plants. You can make sure that your compost remains oxygen rich simply by turning or mixing the pile every week or so, or anytime you notice it becoming odorous.