Yes, they are, and no they aren't.
Yams and sweet potatoes are two completely unrelated plants from different parts of the world that both produce tubers. In the USA, many people refer to orange-fleshed sweet potatoes as "yams" and yellow-fleshed sweet potatoes as "sweet potatoes", but they are really all varieties of the same vegetable (the sweet potato).
True yams are the tuber of a tropical vine that is native to Africa, and they are not even distantly related to the sweet potato. They are common in Carribean cooking, and are becoming more readily available in the States.