The terms "sweet potato" and "yam" are used almost interchangeably in modern America, especially in the south. The two, however, refer to three completely different plants, from three completely different plant families. Despite the botanical differences, however, the way yams and sweet potatoes are cooked and used are quite similar.
Sweet Potatoes
Sweet potatoes (Ipomoea batatas) are a member of the morning glory family. A New World plant, the sweet potato was a staple of the Central American diet when Columbus landed in the 15th century. Sweet potatoes range in color from pale yellow to deep orange. They range in texture from firm to soft and mushy when cooked.
Yams
"Yams" refers to several species of the Dioscorea family. The yam is more closely related to lilies and grass than it is to sweet potatoes. It is a tuber and is covered with a scaly skin that resembles bark. Though a common part of diets through Africa, yams are rarely found whole in North American markets. When they are, it is usually in specialty and ethnic markets, and they are usually either canned or cut up and shrink-wrapped. If you find a whole "yam" in the market, it is most likely a sweet potato, not a yam.
The Confusion
The confusion between the two terms -- yams and sweet potatoes -- began when African-American slaves saw the similarities between the sweet potatoes in America and the yams they used to eat back in Africa. They began calling sweet potatoes "yams," and the name stuck. To further complicate the matter, some sweet potato growers began using the two terms to distinguish between the two main kinds of sweet potatoes. They called the firmer-fleshed, pale yellow sweet potatoes "sweet potatoes" and the softer-fleshed, orange sweet potatoes "yams." And if that isn't enough confusion, in New Zealand, "yams" refers to a third species of root vegetable, Oxalis tuberosa, sometimes called oca. It originally came from Bolivia and Peru and ranges in color from purple to pink to yellow.
Baking Yams
Whatever the species, yams can be baked in their skins like sweet potatoes. American yams, soft-fleshed sweet potatoes, may take more or less time than their firm-fleshed cousins, but they bake up just as well. African yams have traditionally been baked in banana leaves in much the same way as sweet potatoes are sometimes baked in aluminum foil. Even the oca, New Zealand's yam, can be baked like a sweet potato, though it is more commonly boiled.
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