From www.southernfood.about.com
Yam or sweet potato, what in the world is it? Many people use these terms interchangeably both in conversation and in cooking, but they are really two different vegetables.
Popular in the American South, these yellow or orange tubers are elongated with ends that taper to a point and are of two dominant types. The paler-skinned sweet potato has a thin, light yellow skin with pale yellow flesh which is not sweet and has a dry, crumbly texture similar to a white baking potato. The darker-skinned variety (which is most often called “yam” in error) has a thicker, dark orange to reddish skin with a vivid orange, sweet flesh and a moist texture.
The true yam is the tuber of a tropical (African) vine (Dioscorea batatas) and is not even distantly related to the sweet potato. It has tubers which can grow up to seven feet long!
Store: Yams should last for two weeks or more if stored in a cool, dark, well-ventilated place and handled with care.
Prep: Before using, gently scrub sweet potatoes with a cloth under running water, to remove dirt. Avoid using a brush, as it will take the skin off of the sweet potato.
Use: Like potatoes, sweet potatoes are always eaten cooked, but their sweetness makes them versatile. They can be used in a wide variety of dishes, both savory and sweet, and go well with cinnamon, honey, lime, ginger, coconut and nutmeg. Enjoy them in baked desserts and quick breads, puddings and custards, casseroles, stews or croquettes.