BJJ is a form of martial arts; self defence.
BJJ was founded by Mitsuyo Maeda from the Gracie family.
The best grand master is Relson Gracie and that is totally a fact and not my opinion cause he is awesome. (He isn't actually the best, I don't know who is and I don't plan on looking into it anytime soon, but you can if you want.) He may not appear very muscular, but in BJJ muscle is not a necessity. Technique is what matters, jiu-jitsu is all about technique. If you don't have technique, you won't succeed.
There are eight total ranks of belts, (for adults) starting from first to last: white, blue, purple, brown, black, red/ black (coral), red/ white, and red. Grand masters have red belts, and they take many, many years of hard work and dedication to earn. Before you can go up in belt rank, you must get stripes, so it can take over a year to get your next belt, sometimes even more than three. White, blue, purple, and brown belts need a black row and four white stripes before their next belt. Belts/ stripes are given by the instructors, or the teacher, when they believe you have earned it. Getting a new belt is a big accomplishment because it shows how far you've gotten with all you effort you put in to get earn it. There are also classes for kids, or people younger than eighteen, and they have different belt rankings and they earn their stripes and belts a little easier than the adults belts.