Brazilian Jiu-Jitsu (BJJ) is a martial art and self-defense system that focuses on grappling and ground fighting. It was developed from the Japanese martial art of jiu-jitsu and judo by the Gracie family in Brazil in the early 20th century. The primary emphasis in BJJ is on techniques that allow a smaller, weaker person to defend themselves against a larger, stronger opponent by using leverage, technique, and strategy rather than brute strength.

Here are some key elements of BJJ:

1. Ground Fighting: Unlike many martial arts that focus on striking, BJJ emphasizes fighting on the ground. Practitioners learn various techniques for gaining control over an opponent, including joint locks and chokeholds.

2. Submissions: BJJ teaches practitioners how to submit their opponents through various techniques. Submissions include joint locks (which apply pressure to joints to force a tap-out) and chokes (which cut off blood flow or air to force a tap-out).

3. Positions: The art includes various positional strategies, such as the guard (where one person is on their back with their legs controlling their opponent), mount (where one person is sitting on top of their opponent’s torso), and side control (where one person is perpendicular to their opponent on the ground).

4. Flow and Transition: BJJ practitioners learn to transition smoothly between positions and techniques, making it possible to exploit openings and respond to their opponent's movements effectively.

5. Rolling: Sparring in BJJ, known as "rolling," involves live practice with a resisting partner. It allows practitioners to apply techniques in a realistic, dynamic environment.

6. Belt System: BJJ uses a colored belt system to signify a practitioner’s rank and experience level. The progression typically starts with white belt and advances through blue, purple, brown, and black belts, with additional degrees within the black belt rank.

BJJ has gained widespread popularity not only for its effectiveness in self-defense but also for its role in mixed martial arts (MMA), where its techniques are often utilized in competition. It is known for its emphasis on technique, strategy, and adaptability.