Kohlrabi is a sweet, crunchy and versatile vegetable. It keeps very well (for weeks!) and is as good cooked as it is raw. It is also known as a German turnip and I can vouch for its ubiquity in Germany. I grew up enjoying it frequently in one of two preparations–braised and then finished with sour cream, dill and a splash of vinegar or just sautéed in olive oil.
It is not only common in German speaking countries but also in India, especially prominent in Kashmiri cuisine. The leaves are tender and delicious and can be used much like collard greens. It is a brassica just like cabbage, collards, broccoli, etc.
Kohlrabi always needs peeling as the skin is tough and fibrous. The flavor of the root is quite sweet with only a mild cabbage-y flavor. The flesh can get fibrous if the root is stored for a long time or is particularly large although there is a variety called Gigante which is huge and very tender.