Today, Sweet Potatoes.
First, let's take care of all that naming mix-up.
The Yam vs. the Sweet Potato.
In most of the world outside the USA, 'yam' and 'sweet potato' are used in reference to two completely different plants. The yam, or 'true yam,' is an African/Asian native from the monocot family Dioscoreaceae. The sweet potato is a dicotyledon from the family Convolvulaceae. Both are root veggies, though. Both are edible. The true yam is typically bigger and sweeter, and may be found in African specialty grocery stores, sometimes, in the USA.
In the USA, what we call yam and sweet potato are actually different varieties of the same plant: the sweet potato (an Aussie will call it a Kumara). The sweet potato is not in the yam family, botanically speaking, nor is it related to the potato (so good news for all those with nightshade issues).
In the USA, the roots that we usually call sweet potatoes are the varieties that are slightly more dry in texture and less sweet. The roots that we usually call yams are the sweet potato varieties that are moister, sweeter, and usually a brighter color inside. So you can use these different names to help you choose the flavor and texture that you want more.
In the USA, the 'sweet potatoes' sometimes substitute better for potato dishes, especially if you pick a very pale and mild flavored variety like the Yellow Hannah or Golden Sweet. And if you have a dish that has a little natural sweetness, it works better as any sweet flavor that bleeds through won't ruin the dish. They turn out nicely for baked sweet potato fries, as well. 'Yams' may do better for dessert-like dishes or those that you need more moisture, such as mashed, in soups or bready dishes, or fried.
The Japanese sweet potato is very sweet and holds up well when deep fried - this is what you will usually see used in tempera sweet potatoes. The color is dark and the skin is often more purple than other varieties of sweet potato.
In the USA, sweet potatoes may have an edible coating added, like a wax. Grocery stores are supposed to have obvious signage up that will tell you if waxes are used, but in practice it doesn't always happen. Also, some people believe that organic produce cannot have waxes used. This is incorrect. Organic produce simply has to use waxes that are approved for use on organic produce, which can involve food allergens like soy, corn, or casein.
According to this site, the possible edible coatings are:
Semperfresh, Nu-Coat Fo, Ban-seel, Brilloshine, Snow-White and White Wash products (Surface Systems Intl. Ltd.) - these are "Sucrose ester based fruit coatings with sodium carboxymethyl cellulose products manufactured exclusively from food ingredients available in dip or spray. "
Sta-Fresh Products (Food Machinery Corp.) - these are "Natural, synthetic, and modified natural resin products and combinations thereof."
And Fresh Wax products (Fresh Mark Corp.) - these are "Shellac and wood resin, oxidized polyethylene wax, white oil/paraffin wax products."
For those of us with many allergies, having these names may at least help you know what to ask about in re: to your sweet potatoes.