Jiu Jitsu is a japanese (i think) traditional martial art along the lines of Gung Fu and Karate, focusing on the throat strikes and eye gouges and breaking arms and whatnot.

BJJ was a branch off from normal JJ pioneered by the Gracies among others in Brazil (having learned JJ in whatever country it was originated) that focused on what really worked in a fight. They basically decided that ground game was where it was at and wound up for the most part focusing solely on submissions.

However, I think this was a bit flawed, as the average bum on the street, or even average Traditional martial artist (TMAist), has even a basic understanding of striking....hit hard and/or hit fast. Any bum could knock out Ali or Tyson if they got lucky. However, because so many martial artists focused solely on striking or gimmick, complex submissions, they couldn't cope with a solely submission artist. They did so well because they were able to take people out of their game and into theirs. It was the equivalent of taking a poker player and making him play blackjack...both equally difficult to master but entirely seperate. Sure, the poker player may get some lucky cards and win a blackjack game, or vice versa, but on the whole, each player would win his game the majority of the time.