Biodegradation is the decomposition of material by microorganisms. It is often used in sewage treatment.
Biodegradable matter is material that can be biodegraded. For example, most plastic bags are not biodegradable, but paper bags are. In managing waste disposal, this can sometimes be an important difference, because plastic bags stay around forever if left as litter, whereas paper bags eventually decompose. However, in anaerobic landfills, whether a material is biodegradable makes little difference: biodegradable matter usually does not decay, because of the lack of oxygen required by the microorganisms.