The fantastic site HowStuffWorks.com has posted an article
on "How College Admissions Works."
As always with HowStuffWorks, it's a high-level explanation but extensively researched, including interviews with the head of admissions at Duke University.
This is a must-read for every high school student who aspires to go on to college. Lots of good kinks, too to background materials including a bunch of articles about the evolution of the SAT and how it's treated by universities today.