A herbicide is a pesticide used specifically to kill weedy plant species. There are synthetic and organic herbicides. Selective herbicides target a specific group of plants and will not harm others. Broadleaf herbicides such as 2,4-D fall in this group. Non-selective herbicides will kill most anything they contact.
Paraquat is an example.
Pre-emergent is applied before the crop or the weed seeds germinate. Post-emergent is applied after the crop germinates. Pre-plant herbicides are added to the soil before the crop is planted. Proper timing is critical to success. Weed seedlings are easier to treat because they are rapidly growing and succulent.