What’s the difference between a sweet potato and a yam? I see these names used interchangeably at the supermarket.

What are often referred to as yams are actually sweet potatoes, which are not really potatoes at all but rather storage roots, like carrots. True yams belong to another plant family and are seldom found in this country unless you shop at a specialty store. Grown in the Caribbean, Asia, and Africa (where it is called njam or nyami), a yam is pale and starchy, with rough, scaly skin and a texture and flavor more similar to yuca (cassava). Canned sweet potatoes are also often labeled “yams,” since that’s what people in the U.S. call them.

There are at least 16 varieties of sweet potato. Skin color ranges from tan to reddish purple, usually with deeply colored flesh. Sweet potatoes are one of the most nutritious vegetables. A medium-sized one provides about 10 milligrams of beta carotene (a hefty amount), plus vitamin C, some B vitamins, vitamin E, magnesium, calcium, and fiber—even more if you eat the skin. The true yam has no beta carotene and half the vitamin C.

In spite of their sweet taste, sweet potatoes have the same number of calories as white potatoes, about 100 per 4-ounce serving. But they become a lot more caloric when “candied”—laden with sugar, syrup, and marshmallows—a traditional American recipe that dates back to the late 1880s.