A hybrid vegetable is the result of cross-pollination between two genetically different parent plants. Plant breeders develop hybrids to increase disease resistance, to improve yield, or to select for special fruit characteristics such as color, flavor, or shipping quality.
Heirloom vegetables are cultivated forms of crops that have been perpetuated by gardeners who save seed (or propagate by some other means such as taking cuttings) from year to year. Some heirloom vegetable varieties have been around for more than a century! Gardeners have kept these varieties growing for generations because the crops performed well in a particular area or because they have outstanding flavor, unusual color, or other appealing characteristics.
OP stands for open-pollinated, meaning that wind, bees, or other insects, rather than plant breeders, transferred the pollen to fertilize the flowers. While all heirloom vegetables are open-pollinated, not all OP vegetables are heirlooms, since all seed companies offer modern-day varieties of vegetables that have been pollinated by wind or other means.