Yam is the edible starchy tuber of various climbing plants of the genus Dioscorea, used as a staple food in several tropical countries, especially in Africa. The two most important African yam species are the white yam (Dioscorea rotundata), and, the yellow yam (D. cayenensis). Both are native to Africa.
Unlike potato, yams have a rough skin which is difficult to peel when raw. The skin colour varies from brown to light pink. The edible flesh varies in colour from white or yellow to purple or pink in mature yams.
The tubers most often weigh about 2.5 to 5 kg (5.5 to 11.0 lb) each, but can weigh as much as 25 kg (55 lb).
The most common cooking methods for yams are boiling, frying and roasting (on fire or charcoal).
The largest producers are Nigeria, Ghana, Ivory Coast, Benin and Ethiopia (WIKI)
The White Yam
White yam tuber is roughly cylindrical in shape, the skin is smooth and brown, and the flesh is usually white and firm.
The Yellow Yam
The yellow yam has a yellowish flesh and looks somewhat like the white one. The yellow colour is due to presence of carotenoids.
Another popular, widely distributed and edible yam is the water yam (D. alata). There is also the Chinese yam (D. polystachya)
Other Species referred to as yam
The name yam is sometimes used to refer to the moist orange fleshed sweet potato (Ipomoea batatas) which has no relationship with the yam species mentioned above.
In Malaysia and Singapore, taro is referred to as yam. Several other unrelated species like the Japanese purple potato may also be referred to as yam.