"Amid predicable fare of the Thanksgiving table there usually sits a bright orange dish - perhaps topped with marshmallows and brown sugar - that adds some sweetness to an otherwise savory meal. Southerners often refer to this as sweet potato casserole; Northerns might say it's candied yams. In this case, the Southerners win. The orange-fleshed tuber on American plates and in pies is a sweet potato, regardless of what some traditions - and grocery store label - say. 
For starters, sweet potatoes and yams aren’t even related. Sweet potatoes are from the morning glory family and yams are related to lilies and grasses. Yams - a staple in some West African countries - are native to Africa and Asia, while the sweet potato hails from South America. True yams can’t be found in most American grocery store chains at all, but they are stacked in piles in West African and Caribbean ones. .."
Yam vs Sweet potato