It is considered a vegetable. It looks like a small rutabaga on the outside and tastes like a cross between a water chestnut (my produce guy said it is closely related) and a potato.
They are very good in salads. I had a hard time finding one but they had it in the specialty produce section next to things like freaky looking mushrooms from outer space and some sort of strange zucchini/cucumber like thing.
Warning, they do go bad rather quickly once cut. Get the smallest one they have for what you need. Those puppies arent cheap. Oh and they dont freeze well..bleh!