The Brazilian Jiu-Jitsu (BJJ) belt ranking system presents a practitioner with different colored belts to show their increasing levels of technical knowledge.
While BJJ does share it's origins with the more widely practiced sport of Judo, Brazilian Jiu-Jitsu has now adopted it's own unique belt system.
Modern Brazilian Jiu-Jitsu belt ranking system.
The current belt system is a product of the recent evolution of the sport. In the past practitioners who trained in the the sport were only awarded either white or black belts. The coveted black belt was at that time only worn by instructors, whilst a plain white belt was worn by students regardless of technical proficiency.
Notice the dark and light colored belts.
Within modern Brazilian Jiu-Jitsu the overall skill level of practitioners at the same belt level can vary greatly from school to school. As a sport BJJ is unique in that the promotional criteria, is subjective and left totally in control of the local head instructor. Not being bound by “Katas” or an internationally recognized overseeing body, the sport of BJJ often promotes it's practitioners though a live (sparing) demonstration of skill oppose to total number of memorized moves. By limiting progression though the belt colors to a practitioners ability to preform against other competitors at the same belt level has led Brazilian Jiu-Jitsu to have a conservative approach to promotion compared to other martial arts.
Example of a white belt practitioner who won 7 competitions in a row finally earning his Blue Belt after winning the Pan American championships:
Now although the tradition of competition results as the primary route of promotion holds true in most cases...some schools do place a high value on demonstrated technical advancement, this being the case due to an individuals physical inability or desire to compete.
Example of a Purple belt testing for a Brown Belt:
Over all the average time it takes to achieve the rank of Black Belt in Brazilian Jiu-Jitsu varies between the practitioners ability to learn and the standards of the head teacher. Although on average the time frame is between 8 and 12 years with a consistent training schedule of 3 to 4 times per week.