Rutabagas, also sometimes referred to as swede or yellow turnip, originated as a cross between a cabbage and a turnip. They are very common in Scandinavia (the Finns eat more than anyone else) and other cooler European countries.
They are a staple in winter CSA shares and the two most common varieties are Purple Top and Gilfeather (pictured at left). The Gilfeather is sometimes referred to as a turnip though it is a rutabaga. It is very mild and sweet. The more common purple topped varieties have a creamy yellow flesh whereas the Gilfeather is white.
Roasting the roots sweetens them but can also intensify the bitterness perceived by some, whereas boiling (and then mashing or adding to soups) dilutes the flavor. Rutabagas are often combined with other roots–carrots, turnips, potatoes, parsnips, celery root–in gratins, soups or mashes. They add a distinct and pleasant sweetness.
Rutabagas happen to be very high in Vitamin C, contain many minerals and are high in fiber and have antioxidant properties. Here’s an article with more information on their health benefits.