Despite a lot of rhetoric, people still really aren't sure what GMOs are. It can be pretty confusing. Here is an introduction, from GMO Answers:
Typically when people refer to GMOs they are speaking about Genetically Modified Organisms (GMOs), which are crops developed with genetic engineering, a more precise method of plant breeding.
For more than 10,000 years humans have selectively cultivated plants to create new varieties of crops with desirable traits, like being resistant to pests or diseases or being tolerant to herbicides that allow farmers to better control weeds. Throughout history they have used a variety of plant breeding techniques to produce plants with useful characteristics, including selective breeding, mutagenesis and genetic engineering.
Genetic engineering, also referred to as biotechnology, allows plant breeders to take a desirable trait found in nature, like disease resistance, and transfer it from one plant or organism to the plant they want to improve, or make a change to an existing trait in a plant they are developing.
Genetic engineering differs from other plant-breeding techniques by enabling specific, predictable changes to be made to the plant.
The only GMOs commercially available in the U.S. are the following 10 crops: alfalfa, apples, canola, corn (field and sweet), cotton, papaya, potatoes, soybeans, squash and sugar beets.
Read more on GMO Answers.