A GMO is a Genetically Modified Organism. GMOs are organisms which have had genetic alterations made for specific purposes. This is a major area of agricultural biology, the development of disease, mold and insect resistant plants, higher yield crops, and similar functional concerns. In some cases genes are transgenic, the use of genetic structures across species, or specifically modified structures, which may be as simple as turning on or off a gene, or adding new structures to the genome.
Examples of GMOs:
Genetically modified bacteria to produce insulin, human growth hormone, and clotting factors for anemia sufferers
Golden rice (includes beta carotene to reduce Vitamin A deficiency)
GloFish, the first genetically modified animal to be sold as a pet.