In an effort to reward students for taking challenging courses, many high schools weight grades for AP, IB, Honors and other advanced classes. As a quick example, an 'A' in AP Calculus might earn a student 5 points whereas an 'A' in basic algebra would earn the typical 4 points. A student who takes many accelerated courses could end up with a GPA that is well above a 4.0 on a four-point scale. Part of the rationale behind grade weighting has to do with class rank -- high schools don't want a student to be valedictorian simply because he or she took the easiest classes available.
College applicants need to realize, however, that colleges rarely look at weighted grades. Selective colleges definitely want to see those AP and Honors classes, but the admissions officers will compare all students on the same unweighted scale. To further complicate the issue, many colleges will recalculate a student's unweighted GPA so that it includes just core academic subjects such as math, history, English, language and science.
To learn more, check out this article on weighted grades.