Yams are root vegetables, like sweet potatoes, but of the Dioscorea genus—closely related to lilies. While yams can be similar in size to sweet potatoes, they can grow thicker and longer—multiple feet longer! Yams are starchier, drier, and, based on an intensive Google image search, whiter (though they can also have that seductive stale orange hue). What's worse is that true yams are difficult to find in U.S. grocery stores, and when they are, they usually come from the Caribbean or some other exotic place.
But if you listen you'll hear the false yam speak, quietly, I yam not a yam. Only the true yam will identify itself. Lean close and let it whisper gentle into that good ear: I yam what I yam, and that is all what I yam (except what I yain't).