Many people use the term "sweet potato" and "yam" interchangeably, even though they're totally different vegetables that come from different plant families, and are technically not even potatoes!
What is often labeled as a yam in standard grocery stores is most likely an orange-fleshed sweet potato. In fact, 95% of yams are grown in West Africa, and it can be difficult to find true yams in North America outside of a specialty store.
With so many varieties of sweet potatoes, and even more varieties of yams, it can be difficult to tell them apart. But the easiest way is that yams have dry, starchy flesh and dark skin with a texture similar to tree bark, and the ends are usually round. Sweet potatoes have smoother, more uniform skin, moist flesh, and tapered ends. Both sweet potatoes and yams come in a variety of colors, although most sweet potatoes sold in the U.S. are orange, white/beige, red, or purple.
Yams are dry, starchy, mild, and more potato-like than sweet potatoes, and they're usually not very sweet. Sweet potatoes, however, are true to their name: they're sweet! They also tend to have a richer, somewhat creamy texture. And while both yams and sweet potatoes are versatile when it comes to how they're cooked and the dishes in which they can be used, sweet potatoes have the extra benefits of working well in desserts, and not requiring as much butter, oil, cream or sugar to be added to the recipe in order to improve their taste.
Both yams and sweet potatoes are nutritious, and while yams have more fiber, potassium and Vitamin E, sweet potatoes are the winner when it comes to nutrition due to their higher content of calcium, beta carotene, Vitamin A, Vitamin C, and most B vitamins. Sweet potatoes are also lower in calories, carbs and fat.
Although both yams and sweet potatoes have many similarities, it's the sweet potato that shines in both taste and nutrition. So when someone asks you, "Pass the yams, please" at Thanksgiving dinner, you can reply, "Of course! Here are the sweet potatoes!", and let everyone know the difference between sweet potatoes and yams!