An herbicide is a chemical used to kill or control the growth of plants. Some herbicides (such as synthetic auxins like Agent Orange and triazine) selectively kill broad-leaved plants while leaving narrow-leaved plants (i.e., grasses and cereal crops) unharmed. Other herbicides, such as paraquat, kill all plants.
Herbicide use has dramatically increased crop yield worldwide, thus saving millions from starvation. Unfortunately, herbicide use has also caused serious environmental problems, polluting soil and water and causing health hazards for humans and animals. In addition to causing nervous system damage, liver and kidney damage and birth defects via acute poisoning, many herbicides have been found to cause leukemia and other cancers and are also linked to autoimmune disorders.
More subtly, many herbicides mimic the sex hormone estrogen in humans and other animals. Estrogen-mimicking chemicals can affect the development of human fetuses (causing problems such as malformed or underdeveloped sex organs in male children) and have also been found to harmfully alter the fertility and sex ratios of other animal species such as alligators.
From the science dictionary at http://biotech.icmb.utexas.edu/
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