Sweet Potato, White - [kumara (New Zealand - Maori), C. Ipomoea batatas]
These are the standard grocery store white sweet potato. Sweet potatoes (red or white) are not particularly sweet at harvest but are "cured" at 85°F for 4 to 7 days during which some of the starch is converted to sugar. Pictured are 1/2 pound potatoes.

Select firm sweet potatoes with no soft spots. Do not refrigerate Sweet Potatoes. Temperatures below 55°F will make them hard and ruin the flavor. They should be stored loose (not in plastic) and kept away from sunlight. Stored at 55°F to 60°F and 90% humidity they'll keep for several months but in a normal household environment they should be used within a couple of weeks.

U.S. "Yam" - [Red Sweet Potato, C. Ipomoea batatas]
This is not actually a yam but a sweet potato variety with red skin and orange flesh. The term "Yam" was adopted from the African "nyami", the name used for sweet potatoes by Southern slaves. This was to differentiate it from the white sweet potato in commerce. Unlike True Yams they have smooth skin as other sweet potatoes do.

Pictured are a whole 2 pound and a cut piece from a 1/2 pound sweet potato. This "Yam" has orange flesh that is moister and sweeter than that of the White Sweet Potato but all other characteristics are pretty much the same.

Yam (True) - [boniato (Spanish), nyami (African), D. Dioscorea batatas and similar]
Yams are grown in Africa, the Caribbean and Latin America, but not in the U.S. except a few in Florida. Unlike sweet potatoes they have a rough scally skin, and can grow to as much as 150 pounds, though they are generally marketed at only a few pounds.

New Zealand Yam - [oca, oka (Andean), Oxalis tuberosa]
Neither a yam nor a sweet potato but a completely unrelated tuber from the Andean area of South America.

Nagaimo - [Nagaimo, Yamaimo (Japan); Glutinous Yam, Cinnamon vine, Japanese Mountain Yam, Chinese Yam; Ma (Korea); Shan yao, Huai shan, Huai shan yao (China); Dioscorea opposita]
Native to Southern China this yam has been cultivated in Japan from very early times. Known in the U.S. mainly as an invasive weed, this vine with arrowhead shaped leaves grows large tubers below ground and small tubers above ground. It spreads mainly by dropping these small "air potatoes" and rarely flowers.

Nagaimo is roughly cylindrical and may be two or three feet long, but the photo specimen was 24 inches long, 2-7/8 inches diameter and weighed 3-1/4 pounds. The skin is tan and the flesh is white and, unlike other yams, sticky and more than a bit slimy. The two Japanese names, nagaimo and yamaimo (shorter, thicker), are applied depending on the root shape. Details and Cooking.

Violet Yam - [Purple Yam; Ube, halaya (Fillipino); Ratalu (India); Uhi (Hawaii); Dioscorea alata]
This violet fleshed yam, now easily available in Southern California in Asian markets, is native to East Asia. It is very sweet and used mainly for deserts, particularly in the Philippines. In India it is also used for deserts but also appears in mixed vegetable dishes. Cooked, its purple color darkens and intensifies.

Yampi - [Cushcush, Indian yam, napi; Yampi, Yampie (Jamaica); Maona (Peru); Mapuey (Puerto Rico); Aja (Cuba); Cara doce (Brazil). Dioscorea trifida]
Native to the Caribbean and tropical Central America this yam is grown for its starchy roots and used similarly to Cassava. It contains the bitter toxic alkaloid discorene which is eliminated by cooking. The cut ends of the photo specimens were dipped in wax to prevent drying out and shriveling. The largest of the photo specimens was 11 inches long, 2-1/2 inches diameter and weighed 1 pound.

Raw yampi are very mucilagenous but this goes away when cooked. Cooked the texture and flavor are pleasant and much like a waxy potato but more tender and crumbly and noticeably sweeter. While yampi is a very fine edible, it is overshadowed in its growing region by manioc (casava) which produces a higher yield and is more durable in storage.