well, when you're 15/16 you take GCSES. You usually take about nine or ten - mine were English, English Literature, Religious Education, Double Science (that's a less advanced level of science, but I still did Biology, Chemistry and Physics), Classical Civilisation, Spanish, Art, Maths). GCSEs are marked from A* (best) to E (worst). Anything from A*-C at GCSE is considered a decent pass.
Compulsory schooling ends at that point. You can continue for another two years, and you take A levels. You take AS exams at the end of the first year, which count for half the overall mark, and A2s at the end of the second year.
A levels are graded from A to E. These are all passes. There are three more possible grades for A levels, being the lowest scores - U, N, and X. (if you get U, it was unmarked; N, no mark, and X, you didn't sit the paper).
Again, anything from A-C is considered a decent A level pass.
A levels are worth a certain amount of 'points' to a university - some universities will make their offers to you based on actual grades, but some will make their offers based on points. For example, I was offered BBB from Cardiff University, but 280 points from Swansea University (which translates to BBC). Points are generally a bit more flexible because if I got, for instance, ABD, I'd still get my offer from Swansea where I'd fail it for Cardiff.
