A sweet potato is not a yam. A yam is not a sweet potato. A sweet potato isn't even a potato...a Yam is not a potato either.  

A sweet potato has tapered ends and thin, smooth skin and flesh that can range from light beige to burnished orange to purplish, even. A yam is cylindrical, typically white-fleshed (there is a purple variety, too) and has rough, dark, almost hairy skin.  They taste very different too. Yams are starchy and dry. Sweet potatoes are sweet and moist. 

The sweet potato is part of the morning glory family, and yams are related to palms and grasses.  Yams originated in Africa and grow in Africa, Southeast Asia, the Caribbean and Central America. Sweet potatoes grow in North America.