Nearly every material will biodegrade, given enough time. But the length of the biodegradation process is highly dependent on environmental parameters such as humidity and temperature, which is why claiming that a plastic is “biodegradable” without any further context (i.e., in what timeframe and under what environmental conditions) is misleading to consumers.
Compostable plastics are a subset of biodegradable plastics, defined by the standard conditions and timeframe under which they will biodegrade. All compostable plastics are biodegradable, but not all biodegradable plastics would be considered compostable.
While often confused with biodegradable plastics, oxo-degradable are a category unto themselves. They are neither a bioplastic nor a biodegradable plastic, but rather a conventional plastic mixed with an additive in order to imitate biodegradation.
Oxo-degradable plastics quickly fragment into smaller and smaller pieces, called microplastics, but don’t break down at the molecular or polymer level like biodegradable and compostable plastics. The resulting microplastics are left in the environment indefinitely until they eventually fully break down.