With MoM here. Please explain further oh british types.
Tradition; if they can ask it they will out of habit.
For reference:
GCSE (formerly GCE, CSE, and O-level, though they're not exactly equivalent) - exams at age 16.
All school-leavers should have collected at least some of these (English and Maths is essential for virtually anything)
AS-level - exams at 17
A-level (and in my day, but long gone, S-level) - exams at 18.
Used to gain entrance to university.
GCE - General Certificate of Education (originally included O-level and A-level)
O-level - Ordinary level
A-level - Advanced level
AS-level - Advanced Subsidiary level (first part of an A-level, graded separately)
S-level - Supplementary paper, for more advanced students, taken with A-levels but graded separately
CSE - Certificate of Secondary Education (more vocational option than GCE, politically divisive)
GCSE - General Certificate of Secondary Education (merger of CSE and O-level)
Then there are fully vocational qualifications like C&G (City & Guilds) and BTEC (Business & Technology Education Council) qualifications, which are taken in FE (Further Education) Colleges (not equivalent to university) from 16 on, not in school. I suppose these could be considered the next stage on from the old CSE, but they both pre-date and survive it.
Note: Scotland has a different system from the rest of the UK.
Some schools in the UK have started using the IB (International Baccalaureate) instead of the native English system, on the grounds that it is better respected.