GMO stands for "Genetically Modified Organism." They do not occur naturally in nature, and are what some like to call "frankenfoods" of the food industry. They are created by a technique called gene splicing. This experimental technology merges DNA from two or more different species, creating unstable combinations of plant, animal, bacterial and viral DNA. Most all GMO's that are created are made so that they can withstand incredibly high doses of sprayed herbicides and pesticides/insecticides. Some GMO's, such as corn crops, even have pesticides bred directly into the corn itself so that when insects try to eat the corn their stomachs explode and they die. There have been many third party experiments and testings done showing the negative effects that these crops have on the body, yet still GM crops are free to be released to the public in whatever quantity the biotech industry wants.
The majority of developed nations do not consider GMO's to be safe, and it seems that every month more and more countries are either banning or requiring the labeling of GMO products. Currently in the US there is no requirement or restriction of these products, although many bills have been presented by the people to do so. Currently, the best way of voting GMO's out of our food system is at the register. Every time you purchase an item you are voting to keep it GM or not.
Crops with High GMO contamination: Alfalfa, Corn, Beets, Zucchini, Yellow Squash, Canola, Cotton, Papaya, Soy, Rice, Flax, Wheat, Milk, Meat, Eggs, Bee products, and starting in 2016, Apples, specifically the "arctic apple".
Ingredients usually derived from GMO's: Amino Acids, Aspartame, Ascorbic Acid, Sodium Ascorbate, Vitamin C, Citric Acid, Sucrose, Sodium Citrate, Ethanol, High Fructose Corn Syrup, Hydrolyzed Vegetable Protein, Lactic Acid, Maltodextrin, MSG, Textured Vegetable Protein (TVP), Xanthan Gum, Molasses, Yeast Products.