What’s the difference between a sweet 
potato and a yam?

Depending on who you ask…

Although orange-fleshed sweet potatoes have traditionally been referred to as yams in parts of the United States and Canada, they are not part of the same family as True Yams and therefore they are not really yams. True yams are cultivated in Africa, Asia, Latin America, the Caribbean and Oceania and not traditionally in the United Stated or Canada.

In the United States, it’s easier. 

“Sweet potato” is the broad category name, and yams are a type of sweet potato. (So all yams can also be called sweet potatoes, but not all sweet potatoes can be called yams.)