The Advanced Level General Certificate of Education it is now compulsory for intake 2008 and lower years (unless you go to a college). It is split into 2 levels.
Advanced Subsidary Level or AS Level studied in Lower Sixth.
Advanced Level 2 or A2 studied in upper sixth - usally a continuation of your AS subjects.
They are graded on a scale of A* to E like GCSEs with a U for Unrecognised. According to several teachers no one understood the A* grade in the 2009 - 2010 when it was introduced so it caused a mess with University applications.
You usually study 4 subjects you want to do at AS level but drop one so only 3 are studied for the full two years. Along the way a Critical Thinking AS and General Studies A-Level are forced upon you.
These exams are often said to be getting easier by people who have never sat them.