It's important to always use the right tool for the right job. Herbicides can be considered a tool in our garden. As gardeners it's important to understand the groupings of herbicides in the garden.
Pre-emergent herbicides take effect on the plant before it emerges, usually before the seed has a chance to germinate. Corn gluten is an organic pre-emergent herbicide.
Post-emergent herbicides work on weeds after the germination phase. These can be further divided into two categories. Non-selective herbicides kill weeds indescriminately. Selective herbicides kill only specific kinds of weeds. An example of this is 24D, killing broad-leaf weeds while not affecting the lawn.