Brazilian Jiu-Jitsu (BJJ) emphasizes taking an opponent to the ground and using submissions such as chokes and joint-locks to win the confrontation or match. BJJ is based on technique and leverage, not size and strength.
BJJ is a great form of self-defense should the fight end up on the ground. It is also a great competitive grappling sport where one can test his/her grappling skills and it is a fun way to get in shape.
BJJ has been proven to be one of the most effective martial arts in the world.