USDA Organic is the federal-level organic standard in the United States, administered by the National Organic Program. Farms undergo a three-year transition (during which land can't use prohibited substances), annual audits, and rigorous record-keeping. The label is the legal meaning of "organic" on food sold in the U.S.
Many small farms use organic practices without pursuing certification — the cost ($500–$1,500+ annually plus labor for documentation) is hard to justify at small scale, and farmers-market customers often accept the farmer's word over the label. Farms that aren't certified can describe their practices ("no sprays," "IPM only," "grown without synthetic pesticides") but cannot legally use the word "organic" in labeling.