This is an excerpt from The Joy of Composting: A Complete Guide to Successful Backyard Composting by Jim McNelly as posted on the Earth 911 website.

While "germs" and bacteria have earned a negative reputation for causing disease, microorganisms are essential to the natural process of decay and perform the healthful and beneficial service of decomposing dead organic matter and converting it into plant nutrients. The earth would be cluttered with the bodies of dead plants and animals were it not for natural organisms that convert nature's by-products into humus. Compost is a method of creating an environment for these microorganisms to thrive and multiply and therefore, rapidly accelerate the rate of decomposition of your organic material.

Nature does not compost, she mulches. Nature does not build piles of organic matter, mix in water and nutrients, turn the pile, and spread finished compost over the countryside. Nature mulches using thin layers and lets low temperature or "mesophilic" organisms do the decomposition at temperatures up to 120°F. Composting supports a different group of "thermophilic" (heat-loving) organisms that operate at a higher temperature range from 120°F up to 160°F. The organisms themselves generate this heat from the organic matter.

Like the coal, oil, or natural gas that living material could eventually become if it were left to accumulate over time, organic matter contains energy. It is the biology and structure of the compost pile that releases this energy in the form of heat. To compost means to use warm temperature organisms of rapid decomposition to convert organic matter into humus in a matter of months rather than years. To maintain the elevated temperatures, bins or piles must be provided to keep this heat from escaping.

Composting can be understood as a type of bacteria farming. Like other forms of livestock, micro-organisms need food, air, water, and habitat. Food is the organic material. Air is provided by mixing and aerating. Water comes from rainfall and the garden hose. Habitat is provided by the bin or pile that retains heat. Think of a bin as an "oven" when a pile is "cooking". With the proper balance of food, air, and water coupled with sufficient volume to hold heat, micro-organisms will thrive, and the process of turning garbage into gold is in full swing, with the bacteria initiating and sustaining the composting reaction (with just a little help from their friend -- you!). Colonies of bacteria will reproduce of their own accord and convert the food into their own bodies and by-products. The dark appearance of compost is actually billions upon billions of decayed micro-organisms.

In this marvelous process of decomposition, there are organisms at work other than bacteria. An entire ecosystem of molds, fungi, actinomycetes, and others feed upon the matter and the by-products of other organisms. Macro-organisms such as earthworms, nematodes, beetles, insects, and a host of invertebrates graze upon the cultures of micro-organisms. This decomposer food chain is the vital living system that connects the soil with the plant and animal life on the surface.

Nature will eventually recover organic materials whether we mulch or compost them. It is our actions that foster or hinder the natural process. Mulching, passive composting, and active composting are the three means by which we promote the recovery of nutrients and return them to the soil, and the success of our efforts is determined by how we condition the food and provide air and water to aid the rate of decomposition.