London Broil is actually a cooking method, not a cut of meat. The meat is a tougher part of beef that requires it to be marinated well prior to cooking in order for it to be tender enough to cut and chew.
Marinate for at least an hour (I do overnight when I can) using any number of marinades...depends on what you like. Italian dressing, oil and garlic, teriyaki sauce, tequilla and lime all common ideas.
You can pan fry, broil, BBQ, slow cook- its up to you and how you will cook it depends on how you will prepare it. Like I said, the most important part is the marinade. Also, as with any cut of tougher beef, its best to only cook to medium rare and to cut against the grain in thin slices when serving.