Kohlrabi is in the Brassica oleracea category. The part that is eaten is the stem, which swells into a globe, rather than the root. It can grow to the size of a grapefruit, but will tend to have a wooly interior, so it is far better harvested at half the size.
There are two varieties, green and purple, but both taste similar. They have a distinctive flavor, something of fresh water chestnuts with a touch of radish and celeriac. They make a marvelous foil for other flavors; sometimes fooling people into thinking they are artichoke hearts. Kohlrabi is rich in potassium, fiber, and vitamin C.
It is particularly good as a salad or in a platter of crudités when par-boiled. Slice it and simply simmer it for a few minutes, then leave in cold water for a while before draining and mixing with other vegetables.
Grated it can be lightly cooked, stir-fried in a little oil or butter with some mustard seeds to give it some spice. You can stuff it, or use it instead of potatoes in gratin dishes. It can also be used like cauliflower.