According to the USDA National Organic Program:
Organic is a labeling term that indicates that the food or other agricultural product has been produced through approved methods that integrate cultural, biological, and mechanical practices that foster cycling of resources, promote ecological balance, and conserve biodiversity. Synthetic fertilizers, sewage sludge, irradiation, and genetic engineering may not be used.
Certified organic, when used in the United States in relationship to agricultural products, means that the product has been certified to meet specific standards and regulations set by the National Organic Program (NOP). In the United States a true certified organic product is certified by an accredited certifying agent and is allowed to wear the USDA Organic Seal.
Source: National Organic Program.