Sweet potatoes, which are root vegetables, and yams, which are tubers, are two types of herbaceous perennial vines that come from two different plant families native to different locations. Sweet potatoes and yams differ in size, taste, texture and nutritional content.
Sweet potatoes, hailing from the Convolvulaceae family, have white or orange flesh, smooth skin and a sweet flavor. They are typically short and blocky in shape. Sweet potatoes are rich in Vitamins A and C and beta carotene, while yams contain a smaller amount of Vitamin C and more potassium. Yams, from the Dioscoreaceae family, have purple, red or white flesh and a starchy flavor. They have rough, scaly skin and can grow up to 5 feet in length, depending on the variety.
Both yams and sweet potatoes are root vegetables, but yams have a cylindrical shape and come from Africa as part of the lily family. On the other hand, sweet potatoes are related to the morning glory family. Sweet potatoes are elongated in shape with tapered ends and are more commonly found than yams in American grocery stores. Sweet potatoes also feature two varieties: firm and soft.
The firm sweet potatoes have a golden skin and pale flesh. They stay firm after being cooked. The soft sweet potatoes have a copper skin and orange flesh. They tend to change to a creamy and moist texture after being cooked. Soft sweet potatoes may be labeled incorrectly as yams in local grocery stores. In fact, true yams are a rare find.
Firm sweet potatoes were first on the market. When the softer variety began to be produced, the two had to be distinguished from one another. With the soft sweet potatoes slightly resembling yams, stores adopted the name "yam" for them.