Brazilian Jiu Jitsu is a self defense martial art with an emphasis on grappling. In simple terms, BJJ is like wrestling with a karate uniform on. The difference is that the uniform is much thicker to prevent tearing and instead of pinning our opponents, we try to submit them.
Submitting an opponent is done through a series of chokes and joint locks which force an opponent to choose between submitting (tapping you to signify that you are in a dominant position, at which time you release them) or causing them to pass out or suffer injury.
In addition to the self defense side, Brazilian Jiu Jitsu is often practiced as a sport, both on its own and in conjunction with other forms of martial arts, called No Holds Barred, Vale Tudo, MMA, or Cage Fighting.
Brazilian Jiu Jitsu is not reliant on strength or conditioning, though they help, and as a result a very light or relatively weak person can easily practice technique, and win, against a much larger opponent.