A Japanese judoist, prizefighter, and member of the Kodokan (later banned for his prizefighting activities) named Mitsuo Maeda emigrated to Brazil in the 1910's and was helped greatly by a Brazillian politician named Gastao Gracie. In return for his aid, Maeda taught Judo to Gastao's son Carlos, who then taught the art to his brothers, including Helio Gracie, who (with Carlos) is generally regarded as the originator of Brazillian Jiu Jitsu as a style of Judo distinct from the Kodokan's.
Brazilian jiu jitsu became internationally prominent in the martial arts community in the 1990s, when Brazilian jiu jitsu expert Royce Gracie won several Ultimate Fighting Championships against experienced and much larger opponents using the style.
BJJ inherited an emphasis on using offbalancing, leverage, and the opponent's own power, as well as a good deal of technique from Kodokan Judo, especially from the regional variant called "Kosen Judo", which emphasized ground grappling and was heavily influenced by Kito-ryu jujitsu.
Since that time there has been considerable divergence between Judo and Brazillian Jujitsu, although some argue that the difference is more in the culture and the moral goals of the arts than in the physical principles and techniques of the two arts. Factors which contributed to the divergence include the Gracies' desire to create a national martial art, the influence of Brazillian culture, the non-participation of the Gracie schools in sport judo, the postwar closing of the Kodokan (which was only allowed to reopen on the condition that emphasis be shifted towards sport), as well as the Gracies' own additions to the body of technique and opinions regarding self-defense, martial arts and training methods, and, more recently, the influence of mixed-martial-art competitions such as the Ultimate Fighting Championship.
Brazilian jiu jitsu emphasizes ground fighting techniques and submission holds involving joint locks. 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 largely negated if wrestling on the ground; and if either fighter wants the fight to go to the ground, it will. Once the opponent is on the ground, a number of maneuvers (and counter-maneuvers) are available to manipulate the opponent into suitable position for the application of a submission hold.
Submission holds typically involve getting an inescapable grip on an opponent's limb which allows one to pull it to the point where the joint will break if pulled any more. This can cause intense pain, and typically results in the opponent re-assessing their will to continue the fight.
Brazilian jiu jitsu's emphasis on joint locks and maneuvering rather than strikes means that one's technique can be practiced at full speed and full power, identical to the effort and technique used in a real fight. Training partners can resist and counter just as they would in an actual fight, providing valuable real-world experience should the techniques ever need to be applied in an actual fight.
In modern times, many forms of sport fighting have come into vogue. During competition, these styles award points for attacking with certain techniques. For example, a competitor may be awarded 2 points for kicking his or her opponent in the body and 3 points for kicks delivered to the head. Perhaps because of these point scoring systems or simply due to the general ineffectiveness of these forms, these competitions usually turn into games of foot tag with little or no resemblance to actual hand to hand combat.
The main emphasis in Brazilian Jiu Jitsu, both in tournaments as on the street is the same. Dominate the opponent through skillful application of technique and force them to quit (submit) or be knocked out or physically damaged. It is not uncommon to witness a smaller practitioner, male or female, control much larger and stronger opponents and actually force that larger opponent to submit.
Belt ranking in BJJ is quite different from most other forms today. There are only 5 belts as follows; white, blue, purple, brown, and black. Average time to black belt is approximately 10 years. There are no tests to "cram". When you can defeat most white belts, you are awarded your blue belt. When you can defeat most blue belts, you earn your purple belt, etc. This is very different to most other systems where the student is tested every few weeks and awarded their black belt in 1.5 to 2 years without ever testing their actual combat effectiveness against other fighters. This makes for the curious practice within these styles of awarding black belts to children. In Brazilian Jiu Jitsu, the belt is a ranking of a fighter's skill as demonstrated in head to head contests and tournament competition.