A genetically modified organism is any organism that has had its DNA modified in a laboratory rather than through cross-pollination or other forms of evolution. For instance, a bell pepper may have DNA from a fish added to it to make it more drought-tolerant, or a bacterium may have its DNA modified to allow it to metabolize petroleum, for instance to clean up an oil spill.
Genetic modification within agriculture is an issue of some contention in the United States and most other countries; proponents of genetic modification argue that it allows great advances in agriculture (for instance in making plants more tolerant of certain diseases or of water shortages), as well as allowing other beneficial creations such as the petroleum-eating bacteria. People arguing against genetic modification generally argue that the ultimate results of releasing genetically modified organisms are not predictable and may have unexpected and irreversible effects on the environment and that, since genetically modified organisms are patentable under U.S. law, GMO crops can in fact harm agriculture instead, by leaving independent farmers unable to purchase seed each year.
Genetic modification is allowed in the United States on the principle that it has not been proven dangerous; GMO foods are common in the United States and estimates of their market saturation vary widely. Many countries in Europe have taken the opposite position: that genetic modification has not been proven safe, and therefore that they will not accept genetically modified food from the United States or any other country. This issue has already gone before the World Trade Organization, which determined that not allowing GMO food into the country created an unnecessary obstacle to international trade.