Hazardous waste is any kind of toxic chemical, organic, or radioactive waste product that can be harmful if exposed to humans. Hazardous wastes are either reactive, corrosive, ignitabile, and/or toxic, though regulations vary state-by-state as to exact definitions for what is, and isn't, a hazardous waste.
Hazardous wastes are biproducts from a variety of industrial sources -- medical, academic, industrial, military, etc. -- and must be safely managed at every stage of their lifecycle. If released into the environment untreated, hazardous wastes can poison, cause cancer, or induce mutation in living life.