In the USA, there really is no difference. What Americans call yams are really just sweet potatoes. In some regions of the USA, only certain sweet potators get to be called yams, but even there, they're just sweet potatoes.
True yams are almost never found in American markets, and they really don't look much like a sweet potato at all (and the flavor is different too, from what I've heard), so if it looks like a sweet potato or tastes like a sweet potato, it probably is a sweet potato.
Unless you live in a big city, with global, ethnic markets, you're unlikely to ever see a true yam.
Last edited by kaplods; 02-09-2014 at 08:21 PM.