Here's a description from Wiki:
"Kohlrabi (German turnip) (Brassica oleracea Gongylodes group) is a low, stout cultivar of the cabbage that will grow almost anywhere. The name comes from the German Kohl ("cabbage") plus Rübe ~ Rabi (Swiss German variant) ("turnip"), because the swollen stem resembles the latter. The same roots are also found in the German word Kohlrübe, which refers to the rutabaga. Kohlrabi has been created by artificial selection for lateral meristem growth (a swollen, nearly spherical shape); its origin in nature is the same as that of cabbage, broccoli, cauliflower, kale, collard greens, and Brussels sprouts: They are all bred from, and are the same species as, the wild cabbage plant (Brassica oleracea).The taste and texture of kohlrabi are similar to those of a broccoli stem or cabbage heart, but milder and sweeter, with a higher ratio of flesh to skin.