Roundup weed killer is an herbicide currently touted as the most effective on the market according to advertisements. The active ingredient is glyphosate isoproplymine salt. When introduced to a plant's leaves, Roundup takes advantage of the osmolitic action of a plant's vascular system. Essentially, it mimics water. It is taken into the plant's vascular system and passed down to the root, where it inhibits the root from being able to make use of water. There are many alternatives to Roundup on the market, some of which act differently to achieve the same goal.
Roundup falls into this category in that it will kill or harm any plant to which it is introduced. QuikPro is an herbicide composed principally of Diqaut, a contact desiccant which draws the moisture from a plant's foliage. As a result, the foliage dies off, starving the root system. Scythe weed killer uses Pelargonic acid. It acts in much the same way as lead or other heavy metals introduced to the human body. Because of its large chain molecular structure, plants cannot metabolize it and subsequently die in the attempt.
Pre-emergent herbicides kill weeds and other plants before they can sprout. This is achieved by the use of chemical inhibitors. Surflan and Atrizine are both water soluble pre-emergent herbicides sold on the open market. They penetrate plant and weed seeds and react with the seed's primary growth hormone, tubulin, rendering it inert and unable to germinate the seed. Pendimethalin achieves the same goal by inhibiting the seed's cellular division. However, many seeds can survive for years in the ground before dying. Because of this, any pre-emergent herbicide must be used continuously to prevent weeds from sprouting. The benefit of pre-emergent herbicides is they do no harm to plants that are already established.
Post-emergent herbicides are selective herbicides, meaning each given brand is only effective at killing one or two types of weed. This is useful when killing weeds in a well established garden as it will not harm plants you wish to keep alive. Each is keyed to a growth enzyme specific to a given species. Trimec kills broadleaf weeds. Image controls both broadleaf and sedgegrass. Momentum kills buttonweed. Corsair kills fescue. Vantage kills perennial and annual grasses such as might sprout in a garden bed.