The most common preemergence herbicides are synthetic. Natural, nontoxic preemergence herbicides made from corn gluten are safer, though you might have to apply them for several seasons for them to be fully effective. A drawback to these and most other preemergence herbicides is that they kill germinating lawn seed. Check product labels carefully. Postemergence herbicides kill existing weeds that are actively growing.
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.