This article elaborates on the definition of degradable plastics, clarifies the difference between photo-degradable, thermo-oxidative degradable plastics and true biodegradable plastics, and introduces biodegradable materials including starch, cellulose and synthetic bioplastics. It explains the complete biodegradation principle into CO2, water and inorganic salts under microbial action, and interprets the concept of compostable biodegradable plastics, as well as the differences between industrial composting and household composting, including degradation cycles and test standards for biodegradable plastics.