Nor are sweet potatoes actually potatoes. Potatoes are nightshades related to tomatoes and peppers from a botanical point of view while Sweet potatoes are related to morning glory family which comprises over 1,000 species of flowering plants. The sweet potato tubers are moister and have a higher sugar content than yams, hence they live up to their name , they are sweeter. Sweet potatoes tend to be thinner skinned than yams and the end in generally pointed / tapered.
Yams, true yams, originated in the tropics of Africa and can also be found in the Caribbean and South America. They are usually imported to North America although at times they can be grown in some regions.
Like Sweet Potatoes, Yams are not related to potatoes, they are related to lilies. Like sweet potatoes they can be orange or even purple, some varieties are white. The taste differs from sweet potatoes they have a lower moisture content and a higher starch content and are not nearly as sweet.