An herbicide is a product used to kill unwanted plants, much the way a pesticide is used to kill pests. Most often, herbicides are used to kill weeds or to clear and area of brush. There are synthetic and organic herbicides, but they should all be used sparingly and with great care. Just because a product is organic does not mean it can't be toxic to people, pets, livestock, and "good" plants.
There are several types of herbicides available to homeowners and it pays to know both what type of plants you want to get rid of and what type of herbicide you are using.
Some herbicides will kill any plant they come in contact with. Since sprays can often drift while being applied, you can do a lot of damage to your yard and garden with an herbicide.
Broad Spectrum Herbicides
Nonselective or broad spectrum herbicides are used to kill all vegetation in an area. Broad spectrum herbicides do not discriminate between garden plants, lawn, and weeds. Some of the more popular broad spectrum herbicides, like RoundUp and Brush Be Gone, contain a substance called glyphosate. You shouldn't have need for a nonselective herbicide very often and if you do, use it with extreme caution. Studies are still ongoing about the residual nature of some of these herbicides, as well as their safety around humans and other animals.
There are also a few acetic acid based broad spectrum herbicide, like Nature's Glory Weed Killer and Burn Out. These burn the leafy portion of the plant, but in my experience, they rarely kill the roots and the plants eventually regrow.
The best luck I've had with these products is with annual weeds and when treating weeds growing near cement, on a hot, sunny day.
Even these acetic acid products should be used only as the label specifies. Run off can leach into the ground and water supply. As for using regular household vinegar as an herbicide, I haven't had much luck with that as a weed killer at all.
Herbicides can also be selective and used to kill only targeted plants, for example, crabgrass killer. These generally cause some form of hormone disruption in the target plant. Unfortunately we are seeing plants develop resistance to selective herbicides and alternating herbicides can get confusing and laborious.
A pre-emergent herbicide works by preventing seed from germinating, so they are more effective in controlling annual weeds. Preen is a popular pre-emergent herbicide for homeowner use. Corn gluten meal is gaining followers as an organic pre-emergent. Pre-emergent herbicides need to be applied at just the right time. They only work on ungerminated seeds, not existing plants, so they are most effective on annual weeds in the lawn, in garden paths, or in existing beds. Do not apply them where you will be direct sowing. When to apply for which weeds should be explained on the product label.
Safe Use of Herbicides
You should take care when storing and disposing of herbicides, just as you would with any garden chemical. Make certain the bottles are clearly labeled and out of reach of children and pets. I would recommend you have a designated sprayer for herbicides, so that you don't reuse the sprayer with something else, like a fungicide, and spray residual herbicide on your plants by accident.
You should also keep yourself protected. Use rubber gloves when pouring. Goggles are a good idea to protect your eyes, when spraying. And if you get any herbicide on your skin or clothes, clean immediately.
Synthetic or organic, these are all strong substances that should be used sparingly, with care and caution. If the problem is small enough to be eradicated with a little hand weeding, that should be your first course of action. If you absolutely must use an herbicide, for example to get rid of an overgrowth of poison ivy, start with the least toxic option first. Remember, more is not better. Always follow the label instructions.