Herbicides are commonly known as weed killers. They are chemical substances used to control unwanted plants. Selective herbicides control specific weed species and are used to leave desired farm crops relatively unharmed.

Non-selective herbicides (sometimes called "total weed killers" in commercial products) are used to clear waste ground, industrial and construction sites, railways and railway embankments. They kill all plants with which they come into contact.

Herbicides are classified in various ways. They have been grouped by their activity, time of application, method of application, mechanism of action and chemical family.

More Info: en.wikipedia.org