Fall

What is the difference between a sweet potato and a yam?
And here's the answer, according to the Library of Congress:

Although yams and sweet potatoes are both angiosperms (flowering plants), they are not related botanically. Yams are a monocot (a plant having one embryonic seed leaf) and from the Dioscoreaceae or Yam family. Sweet Potatoes, often called ‘yams’, are a dicot (a plant having two embryonic seed leaves) and are from the Convolvulacea or morning glory family.

Food Blogga Translation: Yams and sweet potatoes are different vegetables.

It turns out my local market has gotten it wrong too. What they have been labeling as yams are really red-skinned, orange-fleshed sweet potatoes. Apparently, sweet potatoes' skin and flesh ranges in colors, and they come in "hard" and "soft" varieties. It is the soft varieties, which become moist when cooked, that are typically labeled "yams" here in the United States.