The library of congress puts it best:
"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."
The topic of sweet potato vs. yams was recently covered on an episode of the Food Network's Unwrapped.
Oh Marc Summers, its been a long time since Double Dare...
Thanks to John for bringing this to my attention.