The name "yams" and "sweet potatoes" are often used interchangeably and many people think they are the same or related. They are not the same and, in fact, are not even remotely related.
Sweet potatoes are what you most often see in stores. Frequently they are labeled as yams. Sweet potatoes are oval shaped with a pointed, tapered end. Real yams are more round without the tapered end and can grow much larger.
Yams are sweeter and more moist that sweet potatoes. The confusion began years ago when a sweet potato was developed that was sweeter making it more like a yam. This sweet potato began to be called and labeled as a "yam"...when actually it was a sweet potato.
If that confused you we will now simplify it. You can use either yams or sweet potatoes in this recipe. You probably will not notice much difference either way.