Looking like a knobbly turnip kohlrabi is actually a low stout cultivar of cabbage - indeed the name translates as 'cabbage turnip'. It is a weirdly odd looking vegetable, pale green with strange protruding stems and smooth all over.
Sweet, crisp and juicy it is fabulous grated raw into salads or cut as batons for dips. It is good roasted and is used in indian cuisine cooked in spicy gravy. Don't discard the stems, they taste as good as the bulb and make an interesting substitute for greens.