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 & sweet potato in the US.

Sweet potatoes are sweet (as you can guess from the name) and though they can come in a variety of colors, the ones you'll find in the grocery store are generally orange. True yams are usually white and starchy-tasting and generally larger than sweet potatoes. The two vegetables look a little bit alike, but they're from different plant families.

The tricky part, and the reason I say 'true yam' instead of just yam, is that in many parts of North America sweet potatoes are called yams. For example, when people in the U.S. eat candied yams at Thanksgiving, they're actually eating sweet potatoes.

True yams are common in African cooking, where they're often boiled into stews or cooked and ground into paste or powder. Some Asian cuisine features purple yam, which is a sweeter relative of the true yam used in certain desserts.