from WikiAnswers:
There are approximately 200 different varieties of yams with flesh colors varying from white to ivory to yellow to purple while their thick skin comes in white, pink or brownish-black. Their shape is long and cylindrical (oftentimes having offshoots referred to as "toes") while their exterior texture is rough and scaly. There is great confusion between yams and sweet potatoes in the United States; most of the vegetables labeled "yams" in the markets are really orange-colored sweet potatoes.
The sweet potato has yellow or orange flesh, and its thin skin may either be white, yellow, orange, red or purple. Sometimes this root vegetable will be shaped like a potato, being short and blocky with rounded ends, while other times it will be longer with tapered ends. There is often much confusion between sweet potatoes and yams; the moist-fleshed, orange-colored root vegetable that is often called a "yam" is actually a sweet potato.
Nutritionally speaking:
Sweet potatoes are relatively low in calories and have no fat. They are rich in beta-carotene; having five times the recommended daily allowance of Vitamin A in one sweet potato, as well as loaded with potassium. These nutrients help to protect against heart attack and stroke. The potassium helps maintain fluid and electrolyte balance in the body cells, as well as help maintain normal heart function and blood pressure.
Yams are similar nutritionally but are not a great source of Vitamin A. They are a good source of vitamin C, potassium, manganese, dietary fiber, and vitamin B6 (pyridoxine). Yams are also known to help protect against cardiovascular disease, benefit those with hypertension, and believed to be an anti-cancer food.