Brazilian Jiu-Jitsu is a martial art, combat sport and a self-defense system that focuses on grappling and especially ground fighting. The art was derived from the Japanese martial art of Kodokan judo in the early 20th century, which was itself developed from a number of schools (or Ryu) of Japanese jujutsu in the 19th century.
Brazilian Jiu-Jitsu emphasizes taking an opponent to the ground and utilizing ground fighting techniques and submission holds involving joint-locks and chokeholds also found in numerous other arts with or without ground fighting emphasis. The premise is that most of the advantage of a larger, stronger opponent comes from superior reach and more powerful strikes, both of which are somewhat negated when grappling on the ground.
BJJ permits a wide variety of techniques to take the fight to the ground after taking a grip. Once the opponent is on the ground, a number of maneuvers (and counter-maneuvers) are available to manipulate the opponent into a suitable position for the application of a submission technique. Achieving a dominant position on the ground is one of the hallmarks of the BJJ style, and includes effective use of the guard position to defend oneself from bottom, and passing the guard to dominate from top position with side control, mount, and back mount positions. This system of maneuvering and manipulation can be likened to a form of kinetic chess when utilized by two experienced practitioners. A submission hold is the equivalent of checkmate in the sport. However, it is possible for a combat situation to continue even after a proper submission is performed.