Junior school-maths courses revisit the most fundamental things that have probably been covered in your school in years 4-6. And not only we will revisit them, but also answer all the "why"s and teach how to ask correct questions and properly answer them. I.e we will try to set up mathematical thinking :) Moreover, we will also give a great number of challenging extra-problems for people who find the main content easy.
Junior extra-maths courses are aimed at those who have no or little experience in logical/olympiad-style problems. We will talk about things like "proofs", "counterexample" and see a number of simple but cool mathematical ideas (most of them are olympiad-style). So whether you plan to focus on maths-olympiads, computer science, or you simply want to try out some non-classroom maths, these courses are for you.
Intermediate school-maths courses cover most of the basic and fundamental secondary-school material. Once you are confident with junior-level material feel free to do courses below one by one.
Intermediate extra-maths courses are aimed at those who have about a year or two of experience in solving non-standard/olympiad-style mathematical problems. Knowing all the theorems/facts/ideas/... from Junior level school & extra maths is a prerequisite.
Senior school-maths courses are mostly aimed to give a high-quality introduction into key high-school/university level topics. Once you are done with these courses, secondary-school exams will be ridiculous. Moreover you will be able to solve more than half of final exam questions.
Senior extra-maths courses are for experienced maths-problem-solvers and they last 12 weeks. So apply only if you are prepared to work hard :) The courses are no longer themed; instead they mimic olympiad-training programs given in many countries. Each contains a lot of advanced problems and a number of non-obvious theorems. The courses are most suitable for those who would like to take part in olympiads of serious level or explore hard maths. It is a prerequisite to feel comfortable with all things covered in Junior & Intermediate school & extra courses.