If something is biodegradable, it can decompose naturally. When you're out camping, you shouldn't leave anything behind and certainly nothing that isn't biodegradable. A plastic bottle is the antithesis of biodegradable — it will be there forever.
When you think environmentally friendly, you're thinking biodegradable, and truly environmentally friendly substances degrade quickly into harmless components. What makes something biodegradable is that the decomposition is done by means of living organisms, such as bacteria. When you put earthworms into your compost, they're the living organisms that are assisting you with the decomposition.