I don't know which country's education system you are familiar with, but FWIW the English system is quite simple:-
At the end of year 11, (age 15 or 16) all children sit a set of exams that used to be called ordinary levels and are now called GCSEs. In the past, these exams were timed to coincide with children being old enough to leave school and get a job. They measure a basic level of understanding on a subject by subject basis. From high to low the grade is A*,A, B, C,D,...,G with C being considered the lowest acceptable level for the GCSE to count.
The Conservative Govt, under Michael Gove, tried to get rid of GCSEs because they had become devalued with increasingly large numbers of students getting the top grades, so there was very little differentiation. In the end, he failed but he did manage to make them a little more difficult and he changed the grading system to 9,8,7,...,1 with 9 being the top grade.
The first students will sit these "new" GCSEs in 2017.
Then there are two further years study at the end of which students sit A levels.
Taking 8 or 9 GCSEs is normal, and 3 or 4 A levels. Some do more, some do less, but these are typical numbers.
A Level results are what students use to gain entry to university, or at least they used to. A levels are also being accused of being dumbed down and so many universities now set their own entrance exams.
There are also AS levels, which seem to be a sort of intermediate A level.
(I am not English either btw, this is just what i have worked out form being around).
It is possible to take GCSEs and A levels early, but its unusual and it leaves a question about what to do next. Going to university early is also possible - I went shortly after my 17th birthday, for example - but it does mean that you miss out on some aspects of student culture if you go to early (or too late).