What are the major differences between yams and sweet potatoes?
Actually, yams are often white, and may be purple or other colors..  In the US, what we get labeled as yams are actually sweet pototoes.  (They were similar to the african yams that people were used to, and the name stuck, sort of like how 'pepper' is used for chilies, but they're not even close to the same thing.)

update A longer explanation of the confusion between yam &amp; sweet potato in the US.