Genetically Modified Organisms (GMOs) and Genetically Engineered (GE) foods are food plants or other live organisms that have been altered by changing their DNA in a laboratory. This is usually done to make them more appealing for commercial farmers, such as more disease resistant. The controversy over GMO's is largely around the potential for losing control of the genetic modification and the spread of side effects on the ecology, including humans.
As an example, perhaps the most well publicized GMO is StarLink corn, genetically modified to include an insecticidal protein (Cr9C). This protein kills corn pests in the same way that spray-on pesticides do. The benefit is reduced use of spray-on pesticides. But the protein also has potential to kill other insects, such as Monarch butterflies, and potential health effects on humans. The Starlink protein has been determined to be an allergen to humans. Because of side effects, fields planted in Starlink corn were to be separated from other corn fields. Because of the health effects, Starlink corn has not been approved for human consumption.
Even though Starlink corn was not approved for human consumption, and was grown only for animal feed and industrial uses in fields buffered from other corn fields, it has been found in food products and has been found in fields not planted as such. And even though Monarch butterflies do not feed on corn plants or pollen, they often live near corn fields and corn pollen containing the Starlink protein is often spread onto their food source by the wind.