This question was asked the other day and they are really difference plants. The moist-fleshed potatoes are often called “yams,” but this is a misnomer: The true yam (botanical family Dioscoreaceae) is a large (up to 100 pounds) root vegetable grown in Africa and Asia and rarely seen in the western world. However, common usage has made the term “yams” acceptable when referring to sweet potatoes. The African word nyami was adopted in its English form, yam. This web page has a chart of the difference.

What is the Difference Between a Sweetpotato and a Yam?