A stands for Advanced. As opposed to O Level, which stood for Ordinary. That has now been replaced with GCSE, as you know. A Levels are generally what are required to get into University, although a lot of Universities now accept BTECs etc, which are more vocational and less academic.
A Levels are a 2 year course, but due to the amount of students who dropped out after a year and left with nothing, they now do AS in the first year, and then have the choice of either leaving it at that, or going on and doing A2 in the 2nd year.
AS is considered to be more of an introduction to the subject, and A2 is much harder- hence some students only choosing to do AS. Doing AS still gives you credits towards University entrance, if that is what you choose to do.