Essentially, anything that can be reused by the environment or that can be reused in the natural setting can be composted. The analysis behind composting is that items that would be tossed into a landfill or garbage waste can actually be recycled and enhance the environment. This reduces landfill waste and actually gives life to microorganisms, who feed off of the compost and regenerate energy based off of it. Composted items are placed in a compost bin and what is composted goes back into the natural soil and not in a landfill. Composting thus helps alleviate the problem of pollution and waste, and helps microorganisms live while enhancing the local environment. The more people compost, the thinking goes, the greater the environment is helped.
Composting consists of collecting items that can be crushed and disseminated back into the earth. Items that can be composted are kitchen/ food waste (many leftovers that would be thrown out can actually be composted), grass clippings, hay, weeds, and wood clippings. The common link between all of these is that they are natural and of the earth, containing nutrients that enrich soil, creating a healthy micro- environment for organisms that make their home in the soil. Presumably, these items can be composted (placed in a bin) because of their non- toxin or non- chemically added state, which means being of natural condition, they can be recycled in furthering natural conditions.
If it rots, it can be composted. However, some materials are less that suitable for a compost bin at home because of the processes that they go through before arriving at your home location. White paper is filled with chemicals that make it unsuitable for composting for instance.
Egg shells, vegetable peelings, corn husks, etc. make very good compostables as do grass clippings and brown paper corrugated boxes. Orange, lemon, and other citrus peels and meat products do not compost well and leave a bad smell in your yard as well as attract undesirable animals.
There are several good books on composting, organic gardening and farming etc. My favorites are the Square Foot gardening method by Mel Bartholomew and Lasagna Gardening by Patricia Lanza.
I think the biodegradables are the best to put in the compost bin so that thye can be further used. All those wastes includin the biocomponents not artificial as plastics can put in the garbage. So that though they are of no use they come in use for others.