The Yam and the Sweet Potato are two completely different vegetables. The Yam is not commonly grown in the United States. The confusion comes because the variety of sweet potato grown in the USA is very similar to the Yam grown mainly in Africa. If you are purchasing your potato in the USA it is most likely a sweet potato and the appropriate potato for Thanksgiving dinner.
When it comes to thanksgiving a yam is just a cooked down sweet potato with sugar cinnamon or u can buy them in a can. there is a veggie called a yam but most people are referring to sweet potatoes in the way I described above just ask your grand ma