These are various categories of chemical agents used by man to enhance agricultural production and control disease vectors.

 They are often toxic to non-target organisms when the get into the ecological food chain. Many kill organisms directly or affect their reproductive processes. 
Many persist in the environment (soil, river, lakes, etc). 
variable amount may accumulate in the organisms and transmitted along the food chain and may get more and more concentrated as the food chain ascends. 
They endanger wildlife and man.
 Examples of pesticides are DDT, 2,4-D, Actelic, Karte, Deisi, etc.