A true yam is a member of the Dioscoreaceae family, while the sweet potato belongs to the Convolvulaceae or Morning Glory family. The edible part of the true yam is a large, hairy, starchy tuber that is seldom available in grocery stores and not grown in the U.S. to any great extent since it is a tropical plant. The sweet potato is an enlarged root of the sweet potato plant that is starchy when harvested but becomes sweeter after storage.
True yams are cream colored inside while sweet potatoes are dark orange.
The answer goes back to the 1930's when Louisiana began shipping sweet potatoes to eastern markets. Louisiana was growing the Puerto Rican variety which was sweet, soft and moist when baked. The eastern states were growing the Jersey type sweet potato that had a dry, mealy texture when baked. The term was introduced by African slaves who used it when referring to a sweet potato-like root vegetable they found growing in the West Indies. Before using materials from this site, please read our usage agreement .