Many years ago orange colored sweet potatoes were introduced to the Southern United States. In an effort to distinguish these from the traditional white-fleshed potato, producers called them yams, which is the Anglicize African word, nyami. Today, most of the starchy tubers consumed in the US and labeled as yams are in reality sweet potatoes. Yet yams and sweet potatoes are not the same; in fact, they are quite different from each other.
As far as botanical order is concerned, they are at opposite ends. The sweet potato is a dicot, set in the morning glory family. The sweet potato, whose sweet and moist flesh varies in color from white to yellow and orange, is native to South America; the skin is typically smooth. Garnet, Jewel, and Beauregard are orange fleshed sweet potatoes that often masquerade as yams in the local supermarkets.
Yams, on the other hand, are a monocot, closely related to grasses. In contrast, yams are dry and starchy and rather bland. While yellow or purple in color, the skin is rough and a bit shaggy. Yams are native to North Africa and Asia. They range in size from that of a small potato up to 150 lbs. Yams are a primary agricultural crop in West Africa, where 95 percent of the world's yam crop is grown. Incidentally, both yams and sweet potatoes can be purple.