The Sweet Potato Council's own Head Honcho, 'You Got Him', is up on his tuber history by gum!
YouTube: The reason "yams" and sweet potatoes taste the same to Americans is because they are the same. What producers and grocery stores in the US have been calling "yams" are actually sweet potatoes (Ipomoea batatas).
Real yams (of the genus Diascorea) are not grown commercially in the United States. You probably have to visit an international grocery store to find them.