The true yam is the tuber of a tropical vine (Dioscorea batatas) and is not
even distantly related to the sweet potato. Rarely found in US markets, the
yam is a popular vegetable in Latin American and Caribbean markets, with
over 150 varieties available worldwide. Generally sweeter than than the
sweet potato, this tuber can grow over seven feet in length. The word yam
comes from African words njam, nyami, or djambi, meaning "to eat," and
was first recorded in America in 1676. 

The yam tuber has a brown or black skin which resembles the
bark of a tree and off-white, purple or red flesh, depending on the
variety. They are at home growing in tropical climates, primarily in
South America, Africa, and the Caribbean. Yams contain more
natural sugar than sweet potatoes and a higher moisture content. They are
also marketed by their Spanish names, boniato and ņame. 
Look for potatoes that are fresh, firm, well-formed for their type, and clean with no soft spots, dark spots, cracks or cuts. Eyes should not be sprouting. When buying fresh, specify size. 

High in carbohydrates, potatoes are a good source of protein, potassium, calcium, niacin, iron, phosphorus, vitamin C, vitamin B-6, and dietary fiber. One medium-sized potato contains about 220 calories. Most minerals in the potato are located just below the skin. For optimum nutrition, cook potatoes in their skin, or lightly peel. Eating five daily servings of fruits and
vegetables lowers the chances of cancer. A recent study found that eating nine or ten daily servings of fruits and vegetables, combined with three servings of low-fat dairy products, were effective in lowering blood pressure.

Our information courtesy of this great link to yams and sweet potatoes:
http://homecooking.miningco.com/food/homecooking/library/weekly/aa112497.htm