Postemergence herbicides kill existing weeds that are actively growing. These come in two basic forms: contact and systemic. Contact herbicides kill only the part of the plant they touch. Most act quickly and work best against annual weeds. Systemic herbicides circulate inside the plant, killing the whole thing. They're more effective than contact herbicides on perennial weeds, though repeat treatments might be needed.
You also need to choose between selective and nonselective versions of systemic herbicides. Selective herbicides kill only certain weeds, while nonselective herbicides kill any green, growing plant, whether it's a weed or not. Most broadleaf herbicides, including products like Weed-Away and Weed Warrior, are systemic and selective to kill broadleaf weeds only. They won't kill weedy grasses. Glyphosate—the active ingredient in Roundup and other products—is an example of a systemic, nonselective herbicide that kills broadleaf weeds and weedy grasses. But because it also kills turf and other desirable plants, it's safest to use it on your lawn when you want to kill an entire section and then replant it. Finale, in which the active ingredient is gluphosinate ammonium, is another nonselective used for this purpose.
When using any postemergence herbicide, don't apply them over your entire lawn, if possible. Instead, spot-treat isolated weeds or weedy patches.