Sweet potatoes and yams are actually two very different vegetables.
While American grocery stores use the terms interchangeably, what they are always referring to is actually a sweet potato, not a yam. Garnet Yams and Jewel Yams, for instance, aren’t yams at all — they’re sweet potatoes! Candied Yams? Nope, sweet potatoes. Confused yet?
In fact, chances are that you’ll never accidentally buy an actual yam instead of a sweet potato at your local grocery. You can find “true” yams as well as other varities of sweet potatoes in Caribbean and Asian specialty grocery stores.