Advanced Placement. You take a (supposedly) University-Level course, and at the end of the year you pay to take a test.
It is scored from 1-5. If you get at least a 3, you automatically get College credits(Unless you want to go to a prestigious school, which usually dont honour AP credits) and you get a "Bonus" grade point, allowing you to have a 5.0 Grade Point average.
Regular/College Prep
4=A
3=B
2=C
1=D
0=F
AP/Honours
5=A
4=B
3=C
1=D
0=F
Sure, it's one thing to have a 4.0 GPA by taking regular classes. But having a 4.5 or better says "Yeah, I did well AND took the hard classes". When you apply to Universities, your GPA is a Major part of what they look at.
To put it in perspective, most schools require a 2.0 to be eligible for Sports or Extra-Curricular activities.
At our school, your last Middle School teacher has to approve you for an Honours course (AP difficulty without all of the perks) Freshman year. If your Honours teacher thinks you did well, they sign of your application for next year's classes that you are "Eligible" for AP.
At our school, AP is an elite group, where you have to not only be intelligent, but also fit the personality correctly. As a result, most of the "Jocks" or other "Cool" kids dont make it. There are about 50 AP students in my year, which has a total of about 500 students.
As a result of its difficult nature, AP classes tend to have smaller class sizes, and thus, the learning goes much faster.
I have often found that taking University-Level courses at your local College is substantially easier than AP. But, you have to attend them on your own time, and it is more expensive.