Pesticide is a general term that includes insecticides, herbicides, fungicides, rodenticides or any substance or mixture of substances that can kill, repel, or interfere with a target organism.
Pesticides are chemicals that are intentionally designed to harm organisms and are purposely released into the environment for that purpose. In most pesticide applications, the majority of the pesticide never reaches the target pest, but instead travels into the air, water and soil where non-target organisms are exposed. Pesticides are not a silver bullet, and almost always interfere with the physiology and/or behavior of other living organisms - through the same pathways used for the pest (e.g. most insecticides have broad-spectrum action since animals, including humans, share the same type of nerve cells and neurotransmitters with insects).
Pesticides have been shown repeatedly to adversely impact soil, freshwater ecosystems, amphibians, insects, birds, coral reefs, coral fish, marine mammals, etc. The regulatory process is not adequate to appropriately estimate the ecological risk from pesticides. As the EPA phased out the use of problematic pesticide groups like the organophosphates and carbamates, newer pesticide groups like the neonicotinoids and pyrethroids have increased the toxic load of agricultural lands by 50-fold. These newer products that were intended to be safer have contributed to catastrophic insect and wildlife declines.