Apparently, some still confuse yams and sweet potatoes. I have tracked down most of the history that explains why.
The picture you see now is sweet potatoes.
Click on real yams for a picture of them. You can see they are not sold in most U.S. grocery stores.
Anyone can find out that the tropical yam is a completely different vegetable. African in origin, the most widely cultivated true yam is Dioscorea rotundata. There are pictures of them, but interspersed are pictures of, yes, sweet potatoes.
The sweet potato was a pre-Columbian food of both North and South America and the Carribbean, called batata. It was discovered by Europeans before the recognition of the Incan white potato, the completely different species, but that is the one which became known as the potato, because it was also called batata by the Europeans in the pidgin of the region.
The bright orange-fleshed sweet potatoes (Ipomoea batatas) are relatively new in the U.S. Before introducing these cultivars, the sweet potato had yellowish, pale flesh. At the time of introduction, the new orange varieties were termed "yams" to indicate they were a different thing. Which they were: better tasting, creamier and smoother, and all the other benefits of modern breeding programs in which the U.S. excelled during the 20th century.
Here's a little history of the sweet potato in the U.S.
The mystery of the Polynesian sweet potato.
Here's an odd statement from this page: "The "Jersey" and related varieties having dry mealy flesh are favored in the northern states. The other type, more watery but richer in sugar and more soft and gelatinous when cooked, is favored in our southern states where they are called "yams". I'm not sure I want to try this "Jersey" thing. And it may explain why some of my Northern friends claim to despise sweet potatoes. That, and the overpoweringly sweet preparations of the basically decent orange Southern sweet potatoes -the "candied yam" - served in institutional cooking. You know what I mean. Yech.
Nine varieties of sweet potatoes grown in North Carolina. The purple one is intriguing.
The sweet potato does not have a shelf life as long as regular potatoes. I didn't realize that, although experience should have indicated that as a general rule.
I often cook sweet potatoes in the microwave. They come out best wrapped in plastic wrap. I find they need no salt, butter, or anything. I eat them as is, and I'm usually willing to jazz things up as necessary. I just don't think they need it. I just scrub them clean and poke a hole, although when I haven't, they have never exploded like regular white potatoes may.
I am going to cook some real yams soon. I have never had them.