From Mother Earth News:
Heirloom vegetables, fruits, flowers and herbs are varieties that have remained popular with home gardeners because they grow well and taste great. Loosely defined as plant varieties that have been grown for at least three generations (and sometimes for three or more centuries!), heirloom food plants are varieties that have been selected for their flavor, resistance to pests and diseases, and other traits important to home gardeners. Unlike modern hybrids, heirloom seeds are open-pollinated, which means they will breed true and can be saved by the gardener from year to year — an important consideration for food security and self-sufficiency. Also, heirloom seeds are never genetically engineered.