What’s in a name, wondered the Bard. Turns out that a yam by any other name may very well be a sweet potato! So, here’s a food epiphany for you, that lovely roasted orangey yam you’ve been adding to your soup and stews is actually a sweet potato. If you think this is confusing, don’t blame us. Blame the supermarkets in America where yams and sweet potatoes are often labeled interchangeably. Most of the yams you find on supermarket shelves are actually sweet potatoes. But despite a somewhat similar appearance, there is a marked difference between yams and sweet potatoes. In fact, they are two completely different tubers.
Difference Between Yams and Sweet Potatoes
Yams and sweet potatoes are tubers that come from flowering plants. Some species of yam look similar to sweet potatoes. And that is pretty much where their similarity stops. Yams belong to the genus Dioscorea of the family Dioscoreaceae. It is an umbrella term for many species and native to the warmer regions of the world, mostly Africa and Asia. In many places like West Africa, it is one of the principal agricultural commodities. Yams come in many colors, from white, pink, yellow, to a lovely purple. 
Belonging to the morning glory family, sweet potato is native to tropical parts of the Americas. It is an important food crop in many parts of the world, including the US, the Caribbean, Russia, and Japan. Its culinary importance is long established in countries where it is commonly found. In Japan, it is even used to make alcohol.
Although they are both starchy vegetables that have to be cooked first, one of their marked differences is in their flavors. Sweet potato has a starchy flavor, similar to a potato, but with a sweet taste. Yams have a similar starchy flavor, but turn softer on cooking. Some yams have a more fibrous texture, while some are very soft. Some of them leave a slightly acrid aftertaste that can be reduced during cooking.
So, why the confusion?
The Encyclopedia Britannica puts the source of confusion at the time Africans were brought to America. These slaves called the orangey sweet potato ‘yam’ because of its similarity to the tuber from their homeland. Growers also started using yam to distinguish the orangey sweet potatoes from the firmer, white species. Although the true yam would appear much later, the nomenclature and its consequent confusions were set. 
How to Tell Between Yams and Sweet Potatoes?
Given the confusion, you should know the difference between the two tubers if you intend to cook with either, especially yam. Sweet potatoes have a more slender appearance than a potato, but with tapered ends. They are mainly found in rose, tan, and red colors. The rose-colored potato has the soft orange flesh, while the tan and red have white flesh.
Yams usually have bumpy, rough dark brown skin. You can also get yams with pink and vibrant purple hues. There are many different species of yam. The color of the flesh depends on the species and varies from yellow, purple, pink, and white. Yams also come in many sizes. Some species are similar to potatoes, while some can be as big as 5 feet.
True yams are not as commonly seen in the US. You will find them in specialty stores, especially stores that stock African or Asian produce. We also recommend that you check the label carefully when buying. The US Department of Agriculture mandates that the label ‘yam’ be accompanied by ‘sweet potato’ whenever applicable.
Which is Better?
Sweet potato is rich in vitamin A, B6, and C, potassium, and fiber. Yams, on the other hand, are quite high in protein and vitamin C. But where sweet potatoes may win the fight is their beta-carotene content. Sweet potatoes with purple flesh are also rich in anthocyanins.  
The other advantage that sweet potatoes have over yams is their lower calorific value. Since both are sometimes used as a staple food, the lower calories of sweet potatoes make for a lighter meal. Sweet potatoes also have a lower glycemic index, which means that they release their sugars slowly into the bloodstream.
Can You Use One in Place of the Other?
You may think you’ve been using yams in place of sweet potatoes anyway. But, as we’ve already established, you’ve probably just been using sweet potatoes! So, can the true yam take the place of sweet potato and vice versa? The short answer is, no. That’s because yams have a more starchy, but drier texture. Yams also cover a very wide species, so that you may find bitter versions with acrid flavor as well. Although there are some sweet varieties of yam, they are not as sweet as a sweet potato.