The term biodegradable means "decomposable through the action of biological agents" regardless of whether the organic material is of natural origin (wood, leaves, straw, etc.) or of synthetic origin (material whose structure does not exist in the natural environment, but it is artificially created).
Biological agents are living organisms such as bacteria and other microorganisms, which exert an action of breaking down an organic compound into ever smaller packages (elementary inorganic molecules such as water, carbon dioxide, ammonia and methane) until they become simple inorganic elements (carbon , hydrogen, nitrogen and oxygen) and be available again in the environment as nutrients for other organisms.
The "unpacking" of biodegradable materials into smaller components occurs mainly through processes that consume oxygen (by aerobic microorganisms) or by other processes that produce methane (by some anaerobic microorganisms).
Being biodegradable does not necessarily mean also being compostable, i.e. capable of creating compost (natural fertilizer): the substantial difference between these two characteristics lies in the degradation time of the material.
The European standard UNI EN 13432: 2002 establishes that a material can be defined biodegradable when 90% is disintegrated in 6 months, while it can also be defined compostable when 90% is decomposed in 3 months.