They were at times referred to as yams, but a sweet potato is not a yam although many people use the two names interchangeably, both in conversation and cooking. The yam is not even distantly related to the sweet potato. It’s a tuber that grows on a vine above ground, while a sweet potato is a tuber which is the root of a vine.
To prevent confusion, the United States Department of Agriculture requires that potatoes labeled as "yams" also be labeled as "sweet potatoes."
Sweet potatoes are native to the tropical parts of South America and were used and cultivated there at least 5,000 years ago.