So why the confusion?
According to the US Department of Agriculture, the reason we've come to refer to sweet potatoes as "yams" and vice versa has to do with how certain sweet potatoes were first introduced in America.
For a long time, firm varieties of sweet potatoes apparently were the only ones people in the US were familiar with. Then, when softer varieties were introduced to the masses, African slaves began calling them "yams" because they reminded them of the actual yams that grew on their home continent. Despite being inaccurate, the name "yam" stuck because there was a need to be able to easily distinguish between the firm and soft varieties.
In case you're hungry for some Thanksgiving sweet potato casserole now, here's our recipe.