Genetically engineered foods have had foreign genes (genes from other plants or animals) inserted into their genetic codes.
Genetic engineering can be done with plants, animals, or bacteria and other microorganisms. We have bred plants and animals for thousands of years to produce the desired traits. For example, we produced dogs ranging from poodles to Great Danes, and roses from sweet-smelling miniatures to today's long-lasting, but scent-free reds.
Selective breeding over time created these wide variations, but the process depended on nature to produce the desired gene. Humans then chose to mate individual animals or plants that carried that gene in order to make the desired characteristics more common or more pronounced.
We have been genetically engineering plants since the 1990s. Genetic engineering allows scientists to speed this process up by moving desired genes from one plant into another, or even from an animal to a plant or vice versa.