Genetically modified organisms (GMOs) are living organisms whose genetic material has been artificially manipulated in a laboratory through genetic engineering. This relatively new science creates unstable combinations of plant, animal, bacteria and viral genes that do not occur in nature or through traditional crossbreeding methods. To give you an idea of just how weird this can get, in 1991 a variety of tomato was engineered with genes from arctic flounder to make it frost-tolerant. Fortunately that product was never brought to market, but it is a good illustration of how unnatural GMOs are.
Almost all commercial GMOs are engineered to withstand direct application of herbicide and/or to produce an insecticide. Despite biotech industry promises, none of the GMO traits currently on the market offer increased yield, drought tolerance, enhanced nutrition, or any other consumer benefit.