This section is from the book "Culinary Herbs", by M. G. Kains. Also available from Amazon: Culinary Herbs, Their Cultivation, Harvesting, Curing and Uses.
It may be said that sweet or culinary herbs are those annual, biennial or perennial plants whose green parts, tender roots or ripe seeds have an aromatic flavor and fragrance, due either to a volatile oil or to other chemically named substances peculiar to the individual species. Since many of them have pleasing odors they have been called sweet, and since they have been long used in cookery to add their characteristic flavors to soups, stews, dressings, sauces and salads, they are popularly called culinary. This last designation is less happy than the former, since many other herbs, such as cabbage, spinach, kale, dandelion and collards, are also culinary herbs. These vegetables are, however, probably more widely known as potherbs or greens.