When it comes to selecting the correct weed killer, you first have to make the decision of whether to use a total (non-selective) herbicide, which targets all weeds and grasses alike, or a selective weed killer, which as the name suggests, targets a specific type of weed.
Glyphosate is the former, a broad spectrum total weed killer, and by volume is one of the most popular types of herbicide on the market. In addition to killing weeds, glyphosate is effective at killing all types of plants including perennials, grasses, and woody plants.
Glyphosate is absorbed through foliage and is translocated to the growing points of the weed: the stem, leaves, and roots. Due to glyphosate's mode of action, it is only effective when used on actively growing plants, and as a result, is ineffective as a pre-emergence herbicide.
Glyphosate works by inhibiting an enzyme used in the production of amino acids, which the plant or weed requires to survive. One of the aforementioned amino acids, tryptophan, is required for the synthesis of indoleacetic acid; one of the main contributors to plant growth.
In many cases, additional surfactants are added to herbicide formulations in order to facilitate penetration of the waxy cuticle of the leaves; this increases the potency of the herbicide. As a result, most glyphosate weed killers contain varying surfactants to increase effectiveness.