Yam is the common name for some plant species in the genus Dioscorea (family 'Dioscoreaceae') that form edible tubers. Yams are cultivated for the consumption of their starchy tubers in many temperate and tropical world regions. The tubers themselves are also called "yams", having numerous cultivars and related species.

They are native to Africa, Asia, and the Americas. Yam tubers vary in size from that of a small potato to over 60 g (2.1 oz). Some 870 species of yams are known, and 95% of these crops are grown in Africa.

More Info: en.wikipedia.org