Yams have blackish or brown skin with white, purple or red flesh, while sweet potatoes have white, yellow, red, purple or brown skin. Sweet potato flesh can be white, yellow or orange. Yams are more rich in starch and drier in taste.
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.