Sweet potatoes and yam are two different root veggies. Nutritionally they don't differ much except that sweet potatoes are slightly higher in beta-carotene and vitamin C than yams. And yams tend to be starchier and drier than sweet potatoes. In U.S. grocery stores you will likely distinguish them in a different way: Yam  - with a copper skin and deep orange flesh. Sweet potato - with a golden skin and lighter flesh.