In Japanese, imo means potato. There's a lot more imo variety in Japan than the U.S. - the white potato similar to a round, brown-skinned new potato called jaga-imo (that's the one you see as a pizza topping), the tiny, soft, furry-skinned taro called sato-imo, the long, beige, almost skinless tuber that you grate raw into a slimy topping for soba noodles or raw tuna called naga-imo or yama-imo. But in the context of yaki-imo, the potato in question is always satsuma-imo, the purple-skinned, yellow-fleshed sweet potato that, from time immemorial, has kept Japanese people going in times of famine and want. There are various types of satsuma-imo, some sweet and gooey, others more like a fluffy, starchy Idaho potato with just a hint of sweetness. They come in all shapes, from two-foot long, skinny cylinders to perfectly round balls. I suspect that they grow as a long, bulbous tuber and get cut into potato-sized pieces before being sold, because you will never see one without the cut-off ends that heal over slightly moldy and have to be cut off again before cooking.
Yaki-imo means "baked potato," and the most common way to cook a satsuma-imo is to bake it, which can be done in a home oven but is much tastier when done in a fire or over hot rocks (this last method, called ishi-yaki-imo, is particularly prized). Yaki is one of those Japanese words that's easy to learn but takes a while to get your head around, since it has no direct English translation. It's used for a variety of cooking methods, and covers almost anything except boiling and steaming. Yaki-tori is grilled chicken, yaki-soba is sauteed noodles, okonomi-yaki is cooked like a pancake on a hot griddle, and you also use the word yaki to describe a variety of baked goods like cakes and cookies.
In the winter months, yaki-imo trucks patrol the streets, cooking sweet potatoes in an oven in back and announcing their presence with a recording blasted over a speaker: two long, sad notes followed by a man's singsong voice calling, "Yaki-imo! Oishii yaki-imo!" It sounds very old-Japan, according to older Japanese people, and since the simple song was one of the first Japanese phrases I learned when I lived in Kyoto, it makes me feel nostalgic, too, despite the fact that I never actually experienced old Japan. The yaki-imo man in the photo above parks his truck near a Starbucks in a well-trafficked pedestrian area, and I hear he often gives away two potatoes for the price of one. That was my experience with him, at least. The guys who drive their trucks around blasting their announcement can get a bit tiresome, but now that winter is coming on, I can't wait to hear their sad, sweet, sweet-potato song again. Meanwhile I've been having to yaki my own imo.