British School System??

I'm American, and therefore I am in the American public school system.
I am now writing a story that is set in England, and I know that the system is different there. I have heard about A-Levels, Key Stages, uni(which I'm guessing is university), and sixth form.
All of this means absolutely nothing to me. It's just all so confusing.

In America high school goes:
age 14-15:freshman
age 15-16:sophomore
age 16-17:junior
age 17-18:senior

From there, most continue on to a college for four years (usually).

Can anyone please explain to me the British school system?
Thanks.