...or BJJ for short is, as the name implies, a brand of jiu-jitsu that was developed in Brazil. The founders of the style (members of the Gracie clan) learned Judo from a travelling martial artist from Japan, and then adapted it to their own tastes and needs. It is not completely clear who developed what, but basically, BJJ specialized strongly in groundfighting and included techniques from wrestling into its standup game. Since "Vale Tudo" ("anything goes") tournaments were apperently quite common in Brazil, rules in BJJ reflected this by being more generous than, e.g., Judo rules - more submissions are allowed, even if striking is not part of BJJ tournaments.
BJJ gained world-wide fame through the first Ultimate Fighting Championships held in the US in the early 1990s. Royce Gracie, the champion for his family's style, quickly made it clear to his competitors that without groundfighting knowledge, you can get owned very hard very quickly; he won three out of the first four tournaments, beating opponents significantly stronger than he was. The style spread explosively throughout the US in the following years, but remained relatively unknown in Germany.
Since I had always enjoyed the groundfighting part of Judo, I decided to take the opportunity and went to a BJJ school when I moved to the US. Getting beaten thoroughly by white belts told me I had come to right place.
Although techniques and strategies in Judo newaza (ground technique) and BJJ are similar, there are significant differences: in BJJ, the time limitations are not as relevant. Neither does the referee stand you up if you haven't achieved anything in ten seconds, nor is the fight lost if you struggle in a pin for 25 seconds. Accordingly, the pace is more relaxed, and techniques that take a longer time to unfold are better developed. Particularly, many variations of the guard (controlling your opponent with your legs while on the back) like the Spider Guard are practically unknown in Judo. Also, counters to these techniques - guard passes - are practiced more thoroughly, and a number of additional submissions (foot locks and wrist locks) forbidden in Judo are allowed. The mere amount of time spent on training groundfighting also leads to a generally higher level of sophistication.
I can say little about standup fighting in BJJ, other than that I have barely seen any in six months. Throws seem to be slightly under-developed - the emphasis is on getting your opponent to the ground, no matter whether you slam him violently in the process.
Endless discussions have been held on newsgroups and discussion boards about the merits of BJJ as a self-defense art. If anything like a consensus emerged, it is this: in a situation that involves more than one person, going to the ground on purpose can be a bad idea - it may very well happen anyway, and you are better off if you know what to do then. In one-on-one fights, thorough knowledge of BJJ enables a smaller person to beat an unknowledgeable, larger person - which is what one would expect from any decent martial art, but which not all of them deliver. At any rate, proper BJJ training develops the same qualities as other sports-oriented martial arts, namely endurance, fighting spirit, pain tolerance and the ability to improvise.