The labels "100% organic" and "organic" are regulated by the US Department of Agriculture, which defines the standards that are applied to crops and to livestock in order for these to be labeled as organic.
For crops, these standards include use of organic seeds, avoidance of pesticides, and a prohibition on the use of genetic engineering.
For livestock, the standards include the use of organic feed, avoidance of growth hormones and antibiotics, and year-round access to the outdoors.
The USDA website explains the organic certification program, as well as other labels that are allowed to be attached to food products, such as "Free-range", "Cage-free", "Natural", "Grass-fed", "Pasture-raised", and "Humane". Note that the use of many of these labels however is not regulated. For further information visit the website of the USDA's National Organic Program.
Pesticides in Food
The Environmental Working Group has investigated the issue of pesticide residues on fruits and vegetables. Their "Dirty Dozen" report lists the top fifteen fruits and vegetables with the highest pesticide residue as well as those that are the cleanest. Apples, celery, sweet bell peppers, peaches, and strawberries lead the list of the most pesticide-laden fruits and vegetables. Onions, sweet corn, pineapples, avocados and cabbage are among the cleanest.
Genetically Engineered Organisms
Genetically modified organisms (GMOs) have recently become a topic of considerable controversy. According to the USDA's 2012 report, genetically engineered varieties now account for approximately 88% of corn planted, 94% of cotton, and 93% of soybeans. Approximately 70%-90% of wheat, sugar beets, and canola crops grown in the United States are also genetically engineered.
The Non-GMO project, a non-profit organization that offers verification for non-GMO products, provides information on avoiding GMO foods and list of products that they have verified as GMO-free. Many of these are organic.