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. Crops sustain less damage themselves if they are mature. Sprays and granular forms are most often used.