I was joking before, but seriously:
At the end of year 11 you will be assessed on all the subjects which you are currently studying, through examination. These are your GCSE exams. These results determine whether you will be accepted into any school to do your A-Levels.
A-Levels are what you study over the course of 2 years- year 12 and 13. In year 12, you first select the subjects you want to study which will lead to the course you want to study at university and have AS Examinations at the end of the year which give you predicted grades for the Final Year 13 Exam,(A2 Exams), the 2 because it's your second year doing the course. Universities offer you a place looking at whether your predicted grades for A2 are good enough, and universities may give you an offer, e.g if you obtain grades AAA in your A2 exams then they will accept you for your selected course at university.
Then in year 13, everything leads up to your final A2 Exams for the subjects you'd wished to study at the start of A-Levels. Your result here will determine whether you will go to university or not, and also for what course.