There are two main types of plastic bags: those that are biodegradable are those that are not.
Biodegradable bags are created from a plastic called polylactide, which is produced after starch obtained from corn or potatoes is converted into lactic acid and polymerised. Biodegradation is triggered by ultraviolet light, heat or mechanical stress. As the plastic begins to oxidise it becomes brittle and susceptible to the usual agents of degradation, such as moisture and microbes. When discarded, plastic bags can take as little as a few months to degrade.