Yams? Sweet potatoes? A quick primer

While cooking a white sweet potato for Thanksgiving lunch today, I thought I should figure out, definitively, once and for all, about the different kinds of sweet potatoes and yams.
Yams are the easy part. The confusion arises in that many people, and even many stores (in North America, at least), call some varieties of sweet potatoes "yams", and the two labels are considered pretty much synonymous in many people's eyes. Yams originally come from Africa and parts of Asia, and are related to lilies, palms and grasses. They have a tough, hairy, dark brown skin, rounded ends, and usually a hard creamy white flesh. They are starchy and dry and not particularly sweet to eat. There are some additional varieties of yam, including yellow- and purple-fleshed ones, but the only ones we are likely to encounter here in North America are the brown skinned white-fleshed ones, and even these are quite hard to find. Most of what you see marked as "yams" in the supermarket are actually sweet potatoes. You have probably never actually eaten a yam.
Sweet potatoes are more complicated, mainly because there are more varieties, and these are more widely available. Sweet potatoes originally hail from Central and South America (although many new cultivars have been developed in the United States in recent decades), and are actually in the bindweed/morning glory family, and only distant cousins to the nightshade family (which includes regular potatoes, tomatoes, eggplants and peppers). They come in a variety of smooth skins from tan to brown to copper to red to purple, they usually have tapered, pointy ends, and they sport a variety of different flesh colours and textures, from white to yellow to orange to purple.
The very sweet, soft, orange-fleshed ones that most people associate with sweet potatoes are actually most likely to be jewel, garnet or Beauregard sweet potatoes (although, of course, they may be labelled as "yams"), which are among the more recent American cultivars. They typically have brown to red smooth skins.
And, if you are interested, we had a purple-skinned white-fleshed sweet potato (no idea what the cultivar is), much less flavoursome than the orange-fleshed ones, but very nice all the same mashed with some pepper and butter.