In the US, they are the same thing.
"Definition: This thick, tropical-vine tuber is popular in South and Central America, the West Indies and parts of Asia and Africa. Although sweet potatoes and yams are similar in many ways and therefore often confused with one another, they are from different plant species. In the southern United States, sweet potatoes are often called yams and to add to the confusion, canned sweet potatoes are frequently labeled yams. True yams, however, are not widely marketed and are seldom grown in the United States. Though they can be similar in size and shape to sweet potatoes, yams contain more natural sugar and have a higher moisture content."
(courtesy of foodtv.com)
"Is ice hockey hard? I don't know, you tell me. We need to have the strength and power of a football player, the stamina of a marathon runner and the concentration of a brain surgeon. But, we need to put all this together while moving at high speeds on a cold and slippery surface while 5 other guys use clubs to try and kill us. Oh, yeah, did I mention that this whole time we're standing on blades 1/8 of an inch thick. Is ice hockey hard? I don't know, you tell me. Next question."