Sweetpotato (Convolvulceae) is one word, and as a crop it is totally different from a potato (Solanum tuberosum). The production, harvesting, handling and storing processes are different for these two vegetables.
Sweet potatoes are not related to yams (nyami). So, what's the difference between a sweet potato and a yam?
Sweet potatoes are from the Morningglory (Convolvulaceae) plant family. Yams are from the Yam plant family (Dioscoreaceae). Also, yams are dry, starchy and have white-colored flesh. See photos of yams on this page. They are usually larger (3-8 pounds each) than sweet potatoes.
Here's why some people in places like Louisiana, call sweet potato "yam."
"Several decades ago, when orange-fleshed sweet potatoes were introduced in the southern United States, producers and shippers desired to distinguish them from the more traditional, white-fleshed types. The African word nyami, referring to the starchly, edible root of the Dioscorea genus of plants, was adopted in its English form, yam. Yams in the U.S. are actually sweetpotatoes with relatively moist texture and orange flesh.
Although the terms are generally used interchangeably, the U.S. Department of Agriculture requires that the label "yam" always be accompanied by "sweetpotato." source - Jonathan R. Schultheis and L. George Wilson - Extension Horticultural Specialists, Department of HorticulturalScience, North Carolina Cooperative Extension Service, North Carolina State University. Click here to learn more about the differences between sweet potatoes and yams.