Kohlrabi looks like it came from another planet even though it is a close relative to the broccoli, cabbage and brussel sprout. The kohlrabi is a member of the cabbage (crucifer or mustard) family. The part we eat is the enlarged stem from which the leaves develop.  The enlarged stem is best harvested as soon as it grows to a diameter of two to three inches. Kohlrabi may be white, green or purple in color. Leaves of young plants may be used like spinach, or mustard greens.  Look for kohlrabi bulbs that appear fresh and that are less than three inches in diameter. Leaf stems should be succulent and tender. Large kohlrabi can be woody and tough. 

Kohlrabi is a good source of vitamin C and potassium. It is low in both sodium and calories. One cup diced and cooked kohlrabi contains only 40 calories and 140% of the RDA for vitamin C. 

Small kohlrabi bulbs which are young and tender generally do not require peeling. Medium to larger sizes should be peeled to remove the protective outer skin. The crisp flesh can be served raw in salads, as a relish, or as a crunchy accompaniment to dips. The bulb can be sliced, cut into quarters, cubes or julienne strips and steamed until crisp -- tender. Kohlrabi bulbs can be hollowed out and stuffed with a vegetable or meat filling. 

Here are some Kohlrabi sites:
http://aggie-horticulture.tamu.edu/extension/specialty/kohlrabi.html

http://www.wisc.edu/agjourn/dcfm/farmmarket/kohl.htm

http://www.mpiz-koeln.mpg.de/~rsaedler/schau/vargongylods/Kohlrabi.html