Glyphosate [N-(phosphonomethyl)glycine] is a broad-spectrum systemic herbicide. It is classified as a non-selective herbicide, since it kills most plants, both broad-leaf and grass. It is absorbed mainly through foliage, though also slightly through roots. It works as an enzyme inhibitor, preventing plants from synthesizing the amino acids tyrosine, phenylalanine, and tryptophan.
Glyphosate has quickly become the most popular herbicide in the United States. In 2015, there were over 750 products containing glyphosate for sale in the United States. The market has continued to expand, as more glyphosate resistant crops are engineered, and as more farmers begin using it. It is also popular in other parts of the world, though less than the United States, since many other countries do no permit the production and sale of genetically engineered crops.
There is growing concern over the health impact glyphosate exposure has on humans and other animals. The FDA and other official agencies maintain that there is no serious threat to heath from herbicides, although in some cases, glyphosate is never measured when other herbicide concentrations are tested. Although no definitive proof has been made readily available, researches believe that glyphosate is likely to cause autism and cancer. 
The official decision is that at the levels of exposure are low enough that it does not have a serious impact. However, since the quantity is not being properly regulated, there is concern that it may pose a threat. Additionally, the levels determined to be safe have been discovered to be harmful to certain creatures, such as water fleas (Daphnia magna), even though it is reportedly safe for aquatic invertebrates. It has also been published in the journal Archives of Toxicology that it is "toxic to human DNA even when diluted to concentrations 450-fold lower than used in agricultural applications." A plethera of other claims against the herbicidal agent have begun to emerge, as well.
Monsanto, the company which developed the first glyphosate herbicide (Roundup) has also developed a glyphosate-resistant gene. This gene they then insert into a variety of crops through a process of genetic engineering so that they resist the enzyme inhibitor. These "Roundup Ready" crops now make up the majority of corn, soy, rapeseed (canola), cotton, alfalfa, and other popular crops in the United States.
- Ecotoxicology. 2013 March; 22(2): 251–262
- Archives of Toxicology 2012 May;86(5):805-13