Native to South America, sweet potatoes are an excellent source of vitamins A and C and are a good source of vitamin B6, fiber and potassium. Sweet potatoes come in several varieties, with the pale and dark types being the most common. The pale has a thin yellowy skin with a starchy, crumbly flesh, while the dark (often mistakenly referred to as a yam) has a thick skin and moist orange flesh. Though they can be found frozen or canned, fresh sweet potatoes are best at their height of availability in the winter months. Look for firm potatoes with smooth skin and store unrefrigerated in a cool, dark place.