Sweet potatoes and yams are members of two different botanical families. Sweet potatoes (Ipomoea batatus) come from the tropical areas of the Americas and have a distinctive sweetness, particularly the pink-skinned, orange-fleshed variety, also known as kumara. There are also some sweeter white or yellow-fleshed varieties grown in the tropics.
Though sweet potatoes are native to tropical America, they also found their way to New Zealand and Asia, and hot baked sweet potatoes are sold in winter on the streets of many Chinese and Japanese cities. The Yam (Dioscorea),is a large, brown-skinned starchy root that can weigh over 5 kg. Its bland yellow or white flesh is a perfect partner for spicy stews in the parts of Africa, Asia and the Caribbean where yams are a staple ingredient.
** Asian Recipes