Just as pesticide is used to rid an area of unwanted pests, herbicide is used to get rid of unwanted plant life, like weeds, brush, unproductive bushes or trees, and other growth that takes nutrients away from crops and other useful plants. Many are synthetic and can be toxic to "good" plants, as well as to animals and humans. While there are organic varieties, it is important for people who are using these products to read the labels carefully to ensure that people, pets, livestock and useful vegetation won't be harmed by their use.
Herbicides are available in two main types: nonselective and selective. The nonselective variety is used to kill all growth and is generally reserved for agricultural use or for clearing large or heavily overgrown areas. On the other hand, a selective herbicide is used to target certain types of plant life. This form works to curb growth, usually through some type of hormone disruption, and should not affect other vegetation.
For the home gardener, a nonselective herbicide is rarely required. An organic, selective product is the better choice for maintaining grass and home gardens. Farmers, orchard owners, developers and park services may use non-selective forms to treat large areas and prepare land for future use.
One problem with using a selective product is that it can create resistant species of weeds and parasitic vegetation. It effectively kills most weeds of the particular type is used against, but at least a few plants usually survive. Those that survive can produce resistant offspring.
To avoid creating this resistance, it is best for consumers to use more than one type of herbicide against each specific plant. This can get tricky when attempting to protect some plants while getting rid of others. The best way to avoid resistance, especially with plants that are annuals, is to use the chemical only as recommended and only when absolutely needed. It is also a good idea to use one type during the first application, and another at the next interval so the unwanted plants won't build up an immunity to one particular type.