Yams are just coming into supermarkets now with the supply increasing from June until October. The thumb-length, orangey-pink and yellow tubers that we call yams in New Zealand are not the sweet-potato tropical variety referred to in American cookbooks.
Instead, ours originate from the South American Andes where they are called oca. Slightly tangy, they are great roasted with olive oil (even better with the addition of a little honey or maple syrup).
They can be mashed and are useful in stir-fries. Try them sliced or grated raw into salads. The yellow variety is sweeter than the red. Yams have a crisp texture that turns mealy with longer cooking. They do not require peeling.
Yams are high in fibre, vitamins A and B6, potassium and carotenoid. See yam recipes here.