In order for a plastic to be called compostable, three criteria need to be met:
1. Biodegrade - break down into carbon dioxide, water, biomass at the same rate as cellulose (paper)
2. Disintegrate - the material is indistinguishable in the compost, that it is not visible and needs to be screened out
3. Eco-toxicity - the biodegradation does not produce any toxic material and the compost can support plant growth.
Biodegradable Plastic is plastic which will degrade from the action of naturally occurring microorganism, such as bacteria, fungi etc. over a period of time. Note, that there is no requirement for leaving "no toxic residue", and as well as no requirement for the time it needs to take to biodegrade.
Degradable Plastic is plastic which will undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties. Please note that there is no requirement that the plastic has to be degrade from the action of "naturally occurring microorganism" or any of the other criteria required for compostable plastics.
A plastic therefore may be degradable but not biodegradable or it may be biodegradable but not compostable (that is, it breaks down too slowly to be called compostable or leaves toxic residue).
