I must admit, I've found myself guilty of using the two interchangeably, when, they are actually NOT the same thing. When I began to look on line, I realized many others do the same; in fact, there are websites entitled 'yamsvssweetpotato.com' and so on!
One such website (credit goes to:http://plantanswers.tamu.edu/vegetables/sweetpotato.html) explained that when orange flesh sweet potatoes were introduced in the southern US producers and shippers desired to distinguish them from the more traditional white flesh types. The African word "nyami" referring to the starchy, edible root was adopted in its English form, "yam". Yams in the U.S. are actually sweet potatoes 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 "sweet potato." The following information outlines several differences between sweet potatoes and yams.
The USDA requires that the food label 'yam' must also say 'sweet potato. Sweet potatoes originate from the tropics, but are now grown in Florida; yams are mainly grown in West Africa and Asia. The USDA website suggests one easy way to distinguish what you're getting: 'unless you get your 'yams' from and 'ethnic' store, they are probably sweet potatoes!
Nutritionally, both are great sources of starch; sweet potatoes offer loads of Vitamin A, while yams have only trace amounts.
Personally, when I know I've got a long run or ride coming up the next day, there's nothing I'd rather prepare my body with that a nice, baked sweet-potato-yam with my protein & veg dinner!