What is the difference between sweet potatoes and yams?
The original, "real" yam is relatively uncommon in the United States but popular in Latin America, the Carribean, and Africa. It belongs to the Dioscorea genus of plants.
Sweet potatoes are much more common in the United States, and belong to the Ipomoea genus, which also includes morning glories. (They are only distantly related to ordinary potatoes, which belong to the nightshade family.) Sweet potatoes come in several varieties. Of the two most common sorts, those with thin, light-colored skin and pale, dry yellow flesh are usually called "sweet potatoes"; those with thicker, darker skin and moist bright orange flesh are usually called "yams". Strictly speaking, both are sweet potatoes.
—The Fact Monster