How something biodegrades
When something is biodegradable, or compostable, it means that under the environmental conditions of heat, moisture, oxygen, and microorganisms (bacteria and fungi); the material can break down into biological elements. Biodegradable products are made with polymers that are either derived from plants or are plant polymer synthetics that microbes recognize and can digest. But be aware, these conditions all have to be present in order to do the trick! So, if you throw your trash into a biodegradable bag for the local landfill, then it's likely it won't be able to break down.
Composting at home
If your city does not offer curbside pick-up for compost material, don't take your yard waste and food scraps to the dump! Research public drop-off locations of local composting facilities that can then create healthy, rich soil to sell or to be used for parks and landscape development. Or better yet, make your own!
Food scraps that can be composted are:
Fruits and Vegetables
Coffee grounds and filters
The stained corrugated liners of pizza boxes
Don't be fooled by misleading products, like biodegradable soaps. The way these soaps work is that they do not contain triclosan, triclocarban, or phosphates which are all difficult to biodegrade. However, when your grey water that contains biodegradable soap goes into your drain to enter the sewer system, it mixes with the other non-biodegradable soaps and becomes null and void. Furthermore, if it were to enter our Water environments by itself, the microorganisms present would use vast amounts of oxygen to break the soap down; depriving and suffocating the other beings. The only way biodegradable soaps actually work is by either dumping your grey water into the ground or having it enter your septic tank in your backyard. A good general rule is to collect your biodegradable soapy water and pour it into the ground about 200 feet away from any water sources.
If you find plastics that are advertised as “degradable” or “oxo-degradable”, these do not mean they're biodegradable. They are still made from polymers that microbes have never been exposed to and thus, do not have the proper enzymes to break them down.
What's the diff
The term, "biodegradable" simply means that the product does break down into biological elements...eventually. So, aside from most plastics, this is the case for a lot of our materials, when you think about it. However, "compostable" means that the product also breaks down under the right conditions but has to do so within a specific period of time. Furthermore, a compostable product has to meet the requirements of US Standard ASTM D6400 and the European Norm EN 13432 in order to be considered so. Thus, a biodegradable product could be also compostable, or it might not be. When in doubt, read the fine details listed!