The Difference Between Yams and Sweet Potatoes Is Structural

In the United States, the terms “yam” and “sweet potato” are used interchangeably, but they are completely different vegetables. Yams are starchy and have a rough, brown exterior. They can grow up to 45 feet long and are eaten in parts of Latin America, West Africa, the Caribbean, and Asia. Sweet potatoes are a New World root vegetable, have a softer, reddish skin, a creamier interior, and often, a darker interior. Most American supermarkets are selling you sweet potatoes, not yams. 
The mix-up between yams and sweet potatoes originated from the Trans-Atlantic slave trade