There are two kinds of biodegradable plastics; Bioplastics created from renewable raw materials and plastics made from petrochemical containing biodegradable additives which enhance biodegradation.
Biodegradable does not mean compostable. Biodegradable means that an object can be broken down biologically while compostable means that the process will result in compost.
There is a lot of debate about the environmental impact of such plastics. Whilst some have been proven to break down almost entirely (largely those made from corn and other starches) there is some concern regarding how the other plastics decompose and the materials they leach during this process.