Types of onions include Vidalia onions, scallions, leeks and shallots. Vidalia onions grow only in Georgia. They are very mild, have a sweet taste and don't cause much tearing as the cook prepares them.
Scallions are mild onions with long hollow stems and undeveloped root bulbs. They are good eaten raw or cooked. People enjoy them all over the world and call them several names, including green onion, spring onion, fresh onion or gibbon.
Leeks resemble scallions in that they are long and white on the bottoms, with green leaves. Unlike the scallion, the leek has leaf sheaths wrapped around each other. Gardeners keep the sheaths white by mounding soil or mulch around them. Leeks are either harvested in the summer if they are planted in the spring or are allowed to overwinter. They have a more delicate taste than the usual onion. Cooks use them in stocks and stews.
Shallots have pear-shaped bulbs whose taste is deeper and more intense than other onions. They grow in clusters, so it's common to find two bulbs encased in a copper-colored papery wrapping. Their flesh is white, but tinged with purple. Because their flavor is so potent, cooks often substitute half a measure of shallots to a whole measure of onions.