Xenobiotics are man-made chemical compounds that are very difficult to degrade. These compounds are made by synthetic organic chemicals and are stranger to the biosphere. They accumulate in the environment and cause harmful effects on the living system.
Xenobiotics include chemically synthesized compounds such as pesticides, polyethylene, polystyrene and PVC. Some compounds are recalcitrance i.e., not easily biodegradable due to the extensive branching of the molecule or introduction of halogen, nitro or sulphonyl group.
DDT and halogenated aliphatic and aromatic compounds are most important xenobiotics because they have high potential to damage the environment. Although, many such compounds are highly beneficial to the humans but their long term presence in the environment cause deleterious effects.
The term means transformation of a chemical compound from highly complicated form (organic) to simple (inorganic) form through biological means. If we say a compound is biodegradable, it means that it can be converted into various inorganic forms or can be mineralized i.e., possible to convert into carbon dioxide and water.