What Is a Sweet Potato?

What is a sweet potato and how is it different from a yam? Which should I use?

A:
There are two general types of sweet potatoes sold in most American grocery stores: a starchy pale variety and a bright orange one. Use the pale ones when making whipped potatoes or a dish that should be light and airy. The moister bright orange variety is better for dishes where the potatoes need to hold together, like a gratin.

Neither of these potatoes are "yams," although the name often is used interchangeably with "sweet potatoes." True yams are native to Africa and usually not available in the United States.