Brazilian Jiu-Jitsu (BJJ) is the single most effective martial art you can learn for self-defense. Brazilian Jiu-Jitsu is a martial art/combat sport/self-defense system that focuses on grappling and especially ground fighting. It teaches that a smaller, weaker person can successfully defend themselves against a bigger, stronger opponent by using leverage and proper technique (mostly by applying joint-locks and chokeholds) BJJ is trained for a variety of reasons. Some train for self-defense, some for sport, MMA fighting, or just a fun way to get into shape. Sparring (or rolling) is a big part of training; this ensures that the techniques you learn are applicable in a real world situation.