Varying widely in size, shape, color and taste, heirloom tomatoes are quite different in appearance from common tomatoes. Most are fragile, with few seeds, meaty flesh and a thin skin. This thin skin, however, is what gives the tomato a higher sugar content and excellent flavor. Heirloom tomatoes come with more than one definition, perhaps because there are so many varieties with so much unique history. For certain, they are an open-cultivated cultivar of tomato whose seeds have been passed along from a single crop generation to another. Most importantly, what allows them to maintain the heirloom name is that no genetically modified organism can be used in their production.