A hazardous waste is waste that poses substantial or potential threats to public health or the environment. According to the U.S. environmental laws (see Resource Conservation and Recovery Act) hazardous wastes fall into two major categories: characteristic wastes and listed wastes. 

Characteristic hazardous wastes are materials that are known or tested to exhibit a hazardous trait such as:

• ignitability (i.e., flammable)
• reactivity
• corrosivity
• toxicity

Listed hazardous waste are materials specifically listed by the EPA or State as a hazardous waste. Hazardous wastes listed by EPA fall into two major categories:

process wastes from general activities (F-listed) and from specific industrial processes (K-listed)

unused or off-specification chemicals, container residues and spill cleanup residues of acute hazardous waste chemicals (P-listed) and other chemicals (U-listed)

These wastes may be found in different physical states such as gaseous, liquids, or solids. Furthermore, a hazardous waste is a special type of waste because it cannot be disposed of by common means like other by-products of our everyday lives. Depending on the physical state of the waste, treatment and solidification processes might be available. In other cases, however, there is not much that can be done to prevent harm.

http://en.wikipedia.org/wiki/Hazardous_waste