The sweet potato is a relatively newer food to the western world. A tropical plant, evidence points to it being domesticated some 5,000 years ago in Central America by the Incan and pre-Incan civilizations. From their original name for it, the batata, it is clear how we came to call it a potato.
Nutritionally, they are one of the most efficient foods in existence. Compared to other crops such as corn, soy, and even the Irish potato, a crop of sweet potatoes will produce more pounds of food per acre. Are sweet potatoes good for you? Judging by their high ORAC value, they contain an abundance of antioxidants. Additionally, they have demonstrated anti-inflammatory properties. Over 80% of their protein is in the form of a protease inhibitor, which has been studied for anticancer effects (source). In that study, anticancer effects were concluded both in vitro (test-tube) and in vivo (in-body).
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010