Before 2002, everyone started Sixth Form taking 3 and sometimes 4 'A'-Levels to study for the 2 year duration.
After 2002, the system changed and students chose 4 or 5 'AS'-Levels to study for Lower Sixth (Year 12) then dropped one or two to continue with 2 or 3 in the Upper Sixth (Year 13).
The 'A' in A Level stands for 'advanced'. The 'AS' stands for Advanced Supplementary. It is half an A Level, worth half the UCAS points of a full A Level and the course lasts for 1 year. At college now, students choose 4 or 5 of these AS levels, and drop 1 or 2 at the end of the first year, to carry on with the others to full A Level standard.
The S in AS level actually stands for Subsidiary. About 5 years ago there were also AS levels, standing for Advanced Supplementary, which extended the course - the equivalent of what are now the AEAs (Advanced Extension Awards). For a full A Level (or A2) now you have to take 6 'units', of which units 1-3 make up the AS level. The system is quite (!) hard to explain, but makes some sense when you're actually in it (I just completed my A2s)
Sounds like the system we have had in Scotland for over 30 years - Highers (usually 5) taken over one year and then Advanced Highers (usually 3) taken in the last year.