Whole-wheat pasta is just what it sounds like -- pasta made using the whole grain of wheat, which gives it a slight nutty taste. The outer coat (bran) and inner seed (germ) are ground up along with the grain. Bran is generally dark brown, which is why whole-wheat looks darker than regular pasta.
Because the bran and germ have not been removed, whole-wheat pasta is high in fiber and can be very chewy. If you haven't had whole-wheat pasta before, first try thin pasta, such as spaghetti or linguine, then move on to thicker, somewhat chewier types, such as fusilli or elbow macaroni