Flank, skirt and hanger steak are three different cuts of meat, and each has its own characteristics.
Flank steak is fairly lean (a 3-ounce serving has 158 calories, 6 g fat and 3 g saturated fat), flavorful and not particularly tender. It is best served cut across the grain into thin strips.
Skirt steak is robustly flavorful and rich; a 3-ounce serving has 198 calories, 12 g fat, and 5 g saturated fat. (It is also referred to as diaphragm and Philadelphia steak) and is often used for fajitas. It, too, should be cut across the grain into thin strips.
Hanger/hangar steak (called onglet in France, and sometimes referred to as hanging tender in the US) has not, until recently, been available at the retail level. Even now, it is more common to see it on a restaurant menu than in your local market. Before restaurants in the US "discovered" the hanger steak just a few years ago, it was a "butcher's cut"-- so called because while there was no demand in the stores, butchers liked to take the intensely flavored meat home. There are no official nutrition numbers available yet for the hangar steak—again, because it is not widely available. If you want to try it at home, try Hanger Steak with Green-Garlic Sauce from Sunset magazine.
Recipe: Miso-Marinated Skirt Steak