Interpreting the ranks in BJJ ...
Blue Belt: this is the rank awarded to someone who has a good overall understanding of groundwork. He or she has not necessarily developed any "favourites" yet - but is equally conversant with passing, sweeping, attacking with chokes, armbars and armlocks and has basic escapes from most common positions. All in all, this rank is about a good, overall solid foundation.
Purple Belt: This person has absorbed and understands about three times as much material as the new blue belt. So technically, has many more options in terms of attack and defence. Apart from the extra time and experience on the mat, and having a lot more "arrows in the quiver", the purple belt is someone who us starting to define and conceptualize his or her "game". In other words, this is someone who has worked out a few favourite passes, sweeps and finishes, and knows how most people react to these "favourites" and has a set of plan "B"s. to back them up. So as well as extra "breadth" to their game, they are starting to develop depth.
Brown Belt: The brown belt is someone who has been on the mat for quite some time - 6 to 8 years, on average. And so has a tremendous wealth of knowledge in terms of technique and strategy. What separates the brown belt from the purple is usually the ability they have at solving problems. The brown belt understands the language of bio=mechanics and leverage - they are able to "work through" the problems that arise on the mat, and begin to solve problems regularly, hence, they are at the threshold of being able to coach/instruct well.
Black Belt: Basically, a mature brown belt. Loads of experience, with not only an extremely broad knowledge base, but lots of depth to it as well. By this I mean, that for most techniques, the black belt will understand how people are most likely to react to specific techniques, and has drawn his plans to include these reactions when formulating combinations and instructing others.
I hope that helps with some planning issues - the journey is so much fun; so apart from these guidelines, remember that the most important thing is to enbjoy the training and have fun all the way.
I won't be hitting the keyboard for another week or so - I'm on the operating table later this week, so will slow down till mid next week at least. till, then, train well and avoid too many hyper-extensions.