Below is the difference, also there is confusion where sweet potatoes are called yams and vice versa. You asked.. so you shall receive:

Okinawan sweet potato (white skin/purple flesh): One medium contains 30 g, of vitamin C, 19 mcg folate and traces of other B vitamins. Minerals include 271 mg potassium, 29 mg calcium, 37 mg phosphorus, 17 mg sodium, 13 mg magnesium, and traces of iron, zinc, copper, manganese and selenium. The purple pigment contains high levels of anthocyanin, an antioxidant.

Sweet Potato: Again medium-sized, they provide about 500 percent of your recommended vitamin A; 60 percent of your vitamin D; 45 percent of your vitamin C; 15 percent of your vitamin B-6; 10 percent of your riboflavin; 6 percent of your thiamine, folate, phosphorus and magnesium; 4 percent of your iron, niacin and calcium; and 2 percent of your zinc. Awesome source of potassium.

Yams: High in vitamin C, containing 8.2mg, or 14 percent of the recommended daily intake. Other vitamins include vitamin A, vitamin E, vitamin K, vitamin B6, choline, niacin, thiamin, folate and pantothenic acid. Minerals: potassium and manganese, each with 13 percent of the daily recommended value, calcium, magnesium, phosphorus, selenium, zinc, copper and iron.

And my absolute favourite in the world, the Japanese Sweet Potato: One medium Japanese sweet potato contains 4 percent DV for calcium and iron and contains significant amounts of magnesium, phosphorus, potassium, zinc, copper, manganese and selenium. Sweet potatoes contain B vitamins -- thiamine, riboflavin, niacin, pantothenic acid, folate and B-6, as well as essential amino acids.

Nerd alert! Just because its awesome:

The plant is a herbaceous perennial vine, bearing alternate heart-shaped or palmately lobed leaves and medium-sized sympetalous flowers. The edible tuberous root is long and tapered, with a smooth skin whose color ranges between yellow, orange, red, brown, purple, and beige. Its flesh ranges from beige through white, red, pink, violet, yellow, orange, and purple. Sweet potato varieties with white or pale yellow flesh are less sweet and moist than those with red, pink or orange flesh.

To amaze your friends and provide extra info for future trivia games: In certain parts of the world, sweet potatoes are locally known as camote, kamote, man thet, ubi jalar, ubi keladi, shakarkand, satsuma, imo, batata or el boniato.

Or in New Zealand they are called Kumara. They grow and eat 3 colors, the orange, a yellow skin and fleshed one called gold, and a whitish skin, purple streaked white flesh one that is the best. They are not ever cooked as a dessert.