Yam or Sweet Potato?

A sweet potato is neither a potato nor a yam.

The yellow variety of sweet potato causes the confusion, for it is sometimes marketed as a yam in America to differentiate it from the white variety - but they are both sweet potatoes (genus Ipomoea - family Convolvulaceae; more in common with the morning glory flower than these other foodstuffs). 

However, a yam is a completely different beast (genus Dioscorea - family Dioscoreaceae) and most traditional eaters in the US have probably never eaten one.  

A potato is a Solanum tuberosum of the Solanaceae family (more in common with a tomato or eggplant than either of the other two).

All three may hang together in our consciousness as starchy tubers we like to eat, but that does not make them all hang from the same evolutionary vine.  But being good to eat probably helped make them as populous as they are on the planet.  Being tasty to humans can sometimes be a good survival strategy even if we humans then get them confused with each other.

(pics from wikipedia: top: sweet potato; middle left: yam,  Bottom right: potato)