Thanks for writing. Post-emergence herbicides are products you use on weeds that are already growing; Roundup is probably the most commonly known example.
Pre-emergence herbicides, on the other hand, are products you use that stop weed seeds from sprouting (but don't have much of an effect on weeds that are growing). Preen is probably the most well-known example. Crabgrass preventers are another form of pre-emergence herbicide.
---Justin, Senior Garden Editor, BHG.com