Biodegradable refers to those chemicals or compounds that, once dispersed in the environment, can decompose into elementary substances thanks to the action of bacteria or other microorganisms in an indefinite time.
In order to compost, the product must biodegrade to its basic parts (water, carbon dioxide, and biomass) so that it becomes a fertilizer in all respects. This process happens in a given amount of time and without releasing any harmful residue.
Biodegradability is an essential feature for a product to be compostable, but it is not the only one. In fact all compostable products are biodegradable, but not all biodegradable products are compostable.