No matter how many sugary, orange, marshmallow-topped casseroles you've wolfed down at the holidays, we're willing to bet that you've never eaten a yam. That's because virtually all of the so-called yams consumed in this country aren't what they seem: they're sweet potatoes.
True yams are a starchy tuber that is a staple crop in many parts of the tropics; they are seldom grown in the United States. The vegetables generally sold as yams in supermarkets here are moist, orange-fleshed sweet potatoes, such as the red-skinned Garnet and the brown-skinned Jewel. They contrast with other types that tend to be drier and slightly less sugary, with either yellow or white flesh. (These paler ones, sometimes called Japanese or Cuban sweet potatoes or boniatos, are excellent in stews and stir-fries.) But they are all, in fact, sweet potatoes.
The great yam confusion arose in the 1930s, when growers developed the super-sweet orange-fleshed varieties of sweet potatoes and began marketing them as yams to distinguish them from drier, standard sweet potatoes. (To make matters even more confusing, sweet potatoes themselves aren't even true potatoes ― botanically, they're a member of the morning glory family.) The marketing ploy worked so well that confusion about yams has persisted. Now, however, more stores are labeling ― correctly ― all of these tubers as sweet potatoes, often appending the variety name.
What sweet potato you choose depends on what you're cooking. Because of their high moisture and sugar content, deep-orange Jewels, Garnets, and similar varieties work well in casseroles and gratins. For a simple baked sweet potato or in soups or stews, use whatever you like: a sweet potato by any other name will taste as sweet, or even sweeter.