Yam Tubers

Yam is the common name for some plant species in the genus Dioscorea (Family Dioscoreacae) that form edible tubers. Yams are perennial herbaceous vines cultivated for consumption of their starchy tubers in Asia, Africa, central and south America, and Oceania. The tubers themselves are called "yams". There are many different cultivars of yams.
  In parts of the United States and Canada " yam" is sometimes used to refer to varieties of the completely unrelated sweet potato (Ipomoea batatas).
                   Etymology
The name yam appears to derive from Portuguese inhame or Canadian (Spain) name, which are probably derived from west African languages, such as Fulani, serer, or wolof. The main derivations borrow from verbs meaning "to eat".
                                 Other uses of the term yam
Several other unrelated root vegetables are sometimes referred to as " yams" in the United States. Today, the U.S Department of Agriculture requires sweet potato labeled with the term "yam" to be accompanied by the term "sweet potato".
 Purple sweet potatoes in Okinawa (Oxallis tuberisa) in Newzealand is typically referred to as " yam".
Konjac corms - a Japanese vegetable with similar uses is often colloquially referred to as "yam", although is unrelated.
Taro in Malaysia and Singapore is referred to as " yam".
                                           Description
Yam are monocots, related to Lillie's and grasses. Native to Africa and Asia, yam tubers vary in size from that of a small potato to over 60kg (130lb). Over 600 varieties of yams are known and 95% of these Cross are grown in Africa.
 Yam tubers can grow up to 1.5m (4.9ft) in length and weigh up to 70kg (15olb) and 7.6 to 15.2cm (3.0 to 6.0 in) height. The vegetable has a rough skin which is difficult to peel, but softens after heating. The skin vary in colour from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as "meat". This substance ranges in colour from white or yellow to purple or pink in mature yam.