It's rather hard to explain...
In Secondary School (is that Middle School or High School for you?) we spend 3 years taking most lessons, then at the end of yr 9, we choose the lessons we want to carry on with (ranges from 3 lessons to 6 lessons) which are things like Geography, History, RS long course, Drama, etc. then we have compulsery lessons as well like English, Maths, RS short course, PE short course, etc.
We learn them for 2 years and then take an exam on every lesson which is our GCSE. GCSE's can help us take further education and can get us better jobs. For further education, we go into 6th Form and we choose 4 subjects which we will take as A Levels, normally you can't take an A Level if you haven't got it as a GCSE (so you can't take a French A Level if you haven't got a French GCSE) and some A Levels where there wasn't a GCSE for, you would need multiple ones like for Media Studies, you would need English Literature and maybe Drama, possibly IT.
Then A Levels can get you better jobs again plus with the A Levels you choose, you can then go on to get a College Degree with them. Same things about GCSE's to A Levels applies for A Levels to College Degrees.
Hope that makes more sense instead of less sense...