Junior school-maths courses revisit the most fundamental things that have probably been covered in your school. And not only we will revisit them, but also answer all the why-s and teach to ask correct questions and properly answer them. Moreover, we will also provide a 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. So whether you plan to focus on 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 middle-school material. Once you are confident with junior-level material feel free to just 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, middle-school exams will be ridiculous.
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, with the teachers consider course-X as being really hard by any standard. The courses are most suitable for those who would like to take part in olympiads of serious level or explore hard maths. Apart from feeling comfortable with things covered in Junior&Intermediate courses, participants need to make sure they know enough of the relevant school material.