The kohlrabi bulb grows above ground; it can have green or purple skin (both varieties look identical on the inside). The interior is white, very crisp, almost like a dense apple, and with a sweet, refreshing flavor. If they're very young the bulbs don't need to be peeled; these were older I think (the skin was a little tough) so I peeled them. Absolutely delicious raw (simply sprinkled with sea salt, or to scoop up garlic scape hummus), sauteed, or braised.
Several stems and leaves emerge from each bulb (I'd already cut the leaves off when I took this photo). Young kohlrabi leaves can be steamed or sauteed; the flavor is similar to collard greens (I did a saute/quick braise of the greens from these kohlrabi -- the flavor was good but they were a little tough. A longer braise might have done the trick, but I was impatient for dinner.)
A terrific book for learning everything you ever wanted to know about vegetables (and fruits, grains, seeds, beans, etc.) and their health-supportive properties is The New Whole Foods Encyclopedia, by Rebecca Wood. It's great to have on hand when dealing with new ingredients, and it's just fun to read in general.